As to bleeding current players, I think there is always going to be an issue of turnover. People get bored, their life changes, etc. and they leave. If the replacement rate is low then there is a different issue at play. Retention should obviously be important but it canāt be the only goal. Drawing people into the PvP content is important and needs to target both new PvP players and people who used to PvP. I think solo shuffle did help on this front and revealed a bit about some of the issues in the process.
This might be an unpopular opinion, but I think part of the problem is the type of players they cater to in the PvP side of the game. It is a pretty explicitly competitive space by its nature which means it attracts a more competitive mindset, and those will also be the loudest voices. That same mindset tends to be pretty resistant to change and protective of systems that are based on exclusion rather than inclusion. It may have changed a lot but at the time when I was trying to push into higher end PvP there was a very clear wall where you started queuing into the elite groups and the community suddenly became very insular relative to everyone you had previously played with in LFG. By design of the rating system, their status and rewards benefit from maintaining and only playing within the existing circles. It is actively harmful to themselves and others to help a noob or lower rated player out (not even carrying, but just grouping and trying to teach). In M+ you get a lot of cases where higher rated players will hop in lower keys to teach or help carry a new guild member or something and since there is no opposing side, nobody is harmed by the carry, and since you donāt lose rating for even failing that low key, it is just a time thing for you.
I think there is a fundamental issue with the reward structure in PvP in that it encourages/rewards some of the worst elements of competitive behavior. Why would a player who knows they just simply arenāt going to be able to compete for the rewards based on skill even participate. If someone just simply doesnāt have the reflexes or mental capacity to keep up with PvP at a level necessary to really engage with the reward system then why bother. Even if you do it for the enjoyment of PvP, you arenāt going to enjoy it for long because you are also thrown in a pool full of hyper competitive people who tell you that you donāt deserve to be there because you canāt compete. This might be fine if we were talking about a tournament, but we are talking about a game. The priorities should be broad enjoyment and rewarding engagement above all else and stroking egos last. When skill/achievement is your only measure of merit you alienate a lot of people while simultaneously creating a pretty toxic environment, and that is pretty much all I have seen from PvP in WoW. Blizzard has been improving a lot in the PvE realm as far as making reward systems based on equitable standards rather than elite/skill standards. Some people hate it, but overall I have found the game far more enjoyable with that approach but PvP is still in the stone age comparatively.
If you donāt think this is the case, just compare the reward structure in PvP to any other area in the game, the closest parallel you will get is titles and mounts, but even those arenāt exactly comparable. In PvP you have the rewards are all gated behind rating, which combined with seasonal removal, by design, ensures that every season a portion of the players will not and can never access those rewards. The closest equivalent are Mythic sets which can be collected by anybody who completes the content at any time present or future. If you arenāt able to do it this tier, you can try next tier, or next expansion. PvE mounts are a little closer because they too can go away (something I think needs changed, a saddle system of some sort for doing equivalent content in a different tier), but they are still not based on a rating system that is relative to other players, e.g. your rating goes up when anotherās goes down and there will necessarily always be someone lower than you. You can have every person in a group of 100 players isolated and hit M+ 2500 rating, that is not possible for the same conditions in PvP, there has to be a bottom and top, all of the PvE players could get their achievement/mount, but only a % of the PvP players can get their equivalent. Pretty much the only universally attainable rewards in PvP through sheer effort are the base seasonal mounts and honor/conquest sets. All other titles, mounts, and elite sets are built around maintaining a system that is actively toxic to new players and strokes the ego of the highly skilled players.
Attempts to change these things tend to be met with pretty violent backlash so things are stagnant. Combine that with a general lack of content development, poor balancing, lack of rewards diversity/quantity, antiquated grouping/matchmaking systems, a lack of easily accessible rated content, and it is no surprise things feel bad. I honestly donāt think things can really be fixed under the current rating system because of its punitive nature and the audience it has fostered. If you ditched tying rewards to rating and instead made them currency based then said wins over 2500 get 16x currency, 2200 get 8x, 2000 4x, etc. I bet more players would engage with the content. But that would be a more obviously exploitable system which would upset people and I donāt think blizzard is willing to put in the effort to punish people for exploiting the more equitable systems that they could try. There are a lot of issues in this game that are faced with a similar paradox, you could fix it, but bots, boosters, and the like would exploit it and Blizzard gets money from those subs in addition to costing money/labor to fight them.
My experience with PvP has been that the content that provides decent rewards lacks diversity and elements of pure fun (tanks/cannons are fun but none of the RBGs use that, the closest you get to wacky fun in rated content is Kotmogu buffs), the people that are in the rewarding content tend to be toxic because the only way to succeed is to be overly sweaty about things, the rewards themselves lack diversity and quality, and the best quality rewards from what little there is tend to be gated behind a crowd of the most toxic players and design elements. In addition to all of that rewards and punishments for failure are worse than anywhere else in the game. Fail to get the elite set in the season - gone forever, lose a match - lose cr and get garbage rewards for your time and effort, lose a BG - substantial time waste and awful rewards. Compare that M+ or Raid, wipe on a pull - quick reset and try again, fail a key - rating stays the same and you still get the loot at completion, miss your mythic set - get it during a later tier, excel at all of those content forms - everyone with you benefits, excel in PvP - the other team gets punished and loses. Everything about the design of PvP is pretty toxic to anybody without a very specific mindset, so it isnāt a shock that it is consistently suffering and absurdly resistant to the changes that would make it more popular.
There is also a universal issue to contend with which is Blizzardās development priorities. A thorough balancing patch for each class every few years is not sufficient to keep things healthy. Unless they dedicate people to monthly tuning passes for every class and every part of content, things will always decay. They just donāt seem to want to dedicate the resources necessary to actually maintain a healthy state for even the basic things like classes let alone each PvP bracket.
Edit: That got away from me, didnāt realize I had built such a big wall of text. I might trim it down.