Your premise on new players is not relevant. The game gaining or losing players, and yes it is likely losing them right now due to not having content for over a year, is not relevant to queue times. I can prove this with Math very easily.
You can disagree with me on my belief but given that I can show it to be true using Math (and considering its a simple ratio Math problem it would not even require that much work or be that complex).
Ratios are EVERYTHING. Player count is IRRELEVANT. I will keep my explanation to a middle school level and try to keep my explanation purely analytical and avoid anything too in depth (I do not know how much Math you have had or how long since you have taken Math so I will avoid jumping into high level data analysis).
We are going to use easy numbers here so lets have the DPS count be 50% of the playerbase with Support being 30% and Tank being 20%. Furthermore, lets also explain on the backend what is happening when you queue up (a lot of games like to show this but OW doesnt). You are basically being put in a queue, picture a line at a grocery store with other people who play your role (and presumably at about your SR as well). What this means in practice, given our numbers, is that, provided there are a bare minimum number of players playing in the correct SR, that a Tank queue will be instant and that Supports and DPS will have to wait . What they are waiting for is for the DPS and Supports and Supports ahead of them in line to find a game.
Now here is where I am hoping not to lose you. The NUMBER of people ahead of you in line would fluctuate depending on the playerbase, BUT the ratio itself means that your queue times would remain static.
For an example, let us assume there are 1000 players queueing up per minute who are eligible to be in your game. If you queued as DPS after 5 minutes of building this model, how long would it take you to find a game? By our metrics there are 500 DPS queueing up per minute, 300 Supports and 200 tanks. This means that after 1 minute 100 games of OW have been formed and that 100 Supports and 300 DPS are waiting to find a game. So when we add in the minute 2 people we will have 200 Tanks, 400 Support and 800 DPS and after the games are made 200S, 600D and then with minute 3 you have 200T, 500S, 1100DPS with the matchmaker making another 100 games. If we continue filtering out until minute 5, when you queue up, you would have 200Tanks, 700Support, 2000 DPS. Now to save you some time if you were the first DPS to queue (aka player 2001) you would take a little over 10 minutes to find a game.
Now if we repeat this process with 10x the players at your SR queueing (aka making it 10,000) eligible players and you queue at minute 5 (There is a reason I am waiting 5 minutes to start that I could go into if you want but suffice to say it is important to let the system build itself before taking a measurement and there is a reason I am not letting it go too far as well). If I were to go through the same explanation you would just have to add a 0 to all the numbers and what would happen…you would still have a game after 10 minutes. The wait time DID NOT CHANGE.
RATIOS are ALL that matter when this sort of system. The only places where population are an issue are sub 1k SR and 4100+ SR as those populations have almost had queue time issues.
OW could use an infusion of new players but that will not affect queue times unless those new players want to play an entirely different role than the existing players want to.