The M+ rating system, Raider io addon, Ilvl, and all the above addons all fail at showing skill level. It is purely a generalized rating that says absolutely nothing about an individual player other than completion.
There is absolutely no risk in this rating system, you never lose rating, only gain rating. If there was a risk that your rating would drop, there would be much less people leaving, because leaving at 5 minutes after a wipe is MUCH more detrimental to your rating, then completing an untimed key.
I highly suggest a loss of rating with different losses based on -3 timer, -2 timer, -1 timer, and a gain for +++3 timer, ++2 timer, ++1 timer, and a more severe loss of rating for leaving. If there is a leaver for Any reason at all, the 4 others in the group should not receive a penalty in rating at all.
That would be sufficient in the short term, But in the long term Rating should expand on MUCH more than completion, Things like DPS, HPS, Damage taken as dps, Damage prevented as tank, Interrupts, and a combination of all elements that make a great player.
Just one Example : A HIGHLY rated DPS, should be able to maintain high dps, with low damage taken, and with a large amount of interrupts.
TLDR;
My take away, There is not a single game out there without risk to rating in terms of competitive gameplay. People Doing high keys have zero risk to their rating, people who leave have no penalty, people who donât do well, or even try have zero risk to their rating. A system that incorporates Loss, makes the environment surrounding mid to high keys a much better experience because it will prevent leavers, it will keep rated players in their respective rated tier and that alone will prevent toxicity. People who want to climb out of sub 1000 rated mythic rating will either be stuck there (casual gamers) or move higher into more competitive gameplay.