It would certainly make finding one's true ELO a lot less of a grind, as losses with an A or S rank would be much less painful, LP-wise.
Do you think the fusion of Mastery and ELO is intended, possible, or practical?
The problem is that you'd inevitably end up with an unfair system. If your algorithm supports certain playstyles or champions over other ones (and it always will, or at least always has), then you end up giving a boost to those players even if what they're doing doesn't actually help them win more often than a lower-rated playstyle or champion.
The advantage of a win/loss system is that it's perfectly fair. If you do things that help you win more often, you will be rewarded exactly the same as someone else. If you don't, you're punished just as much as everyone else. It takes a bit longer, but it's fair.
If you can find a ranking algorithm that perfectly rates players based on their in-game actions, then using champion mastery to influence LP would be fine. Until then, it's a bad idea.
As a support main, I will steal every kill if I am not queued up with my ADC.
Do you see where this system is flawed? It promotes greed over teamwork.
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It is not a good idea because your rank does not accurately measure your influence on the game's outcome.
