Well, let's look at some of the issues that directly pertain to restricting players based on Champion Mastery;
- Requiring Champion Mastery will increase the required bar for entry. As it stands right now, you only need to be Level 30 and have at least twelve to twenty Champions unlocked in order to play Ranked (the lower prerequisite for Flex 3's, the higher for 5's and Solo/Duo.) If you only allow Champions of MR5 or higher into Ranked, you consequently need to bump up those prerequisites to require players have 12-20 Champions at Mastery Rank 5, consequently making it needlessly more taxing to get into Ranked.
- Restricting a player's Champion pool restricts their ability to adapt. There are a fair number of Champions with similar playstyles, even if some of them inhabit completely separate roles from one another, and that actually makes this more of a problem when you take into account AutoFill, which will naturally exacerbate that issue. What if I play engage-heavy Support Champions (like Leona), and get autofilled Jungle? I know Diana has a strongly similar kit, but because I don't have her at MR5+, I'm relegated to play some other Champion - implying I even have any Junglers at MR5+.
And that's not limited to just Champions of similar playstyle. If I get autofilled mid lane without any midlaners at MR5, I'm pretty much screwed - or worse, if I do have Champions at MR5+ and the matchup becomes completely unfavorable to those Champions. Alternatively, I could have just two Champions (let's use Yasuo and Azir for example, as I have them at MR7), wherein one is banned (Yasuo) and the other is beyond my ability to play or otherwise not actually good to play.
Which, that brings me to some other issues pertaining to Champion Mastery-based restriction;
- Champion Mastery is not accurately indicative of player skill with a given Champion. Especially if the bar is anywhere below Mastery Rank 6 - Champion Mastery is only indicative of a player's dedication to playing a given Champion, and after that, their ability to get the rare S-/S. As an example, I have Azir at Mastery Rank 7; but I guarantee you I would not do well if I played him today. Partially because of his natural balance instability as a high-mastery curve Champion, but also because I've accumulated so much rust on him that it'd be weeks before I could actually lane halfway decently with him today.
- Champion Mastery also does not account for overall mechanical knowledge. No matter how readily you would regard first-timing a Champion as a heinous blunder, that the implications of being completely new to a Champion could increase the odds of losing, you have to account for people with the mechanical knowledge and general ability to play any Champion reliably well, first-time or otherwise. For some people (and I would hazard to put myself forward as one such player), acclimating to a new and unfamiliar kit is barely an issue, and one could easily do well without prior playtime with the Champion.
Of course, some such cases could simply be blunders of unusual luck or attributed to the Champion being new to everyone, but regardless, the point is the same; some people can adapt to a new kit within the first match, and it would be a disservice to restrict them to playing only Champions they've spent enough time on to bring to Mastery Rank 5 or higher.
All told, I'm personally against imposing a Champion Mastery restriction on Ranked play. It'd be needlessly inhibiting, and it still wouldn't make the situation of some players not knowing their Champion any better.
IMO, if you really want to play with players who know their Champion well and won't first-time something in Ranked, I think you'd be best off getting a premade together to queue for 5's. But that's just my perspective.
