Players using multiple champions is a desirable outcome though, so allowing players to bypass that is counter productive to one of riots goals.
On top of that, your stated goal wasn't for making mastery chests easier, it was to reward champion mastery.
8 games per chest*48 potential chests is 384 games to get all possible chests (slightly less due to the first 3), which is more than enough to reach diamond. What's more, ranked, unlike the weekly mastery chests, has no limitations on how much you can gain in a given period, so theres no 'wasted' games, while someone could be earning S ranks without chests available. As such, the time requirement to earn diamond through single champion mastery is strictly less than the time requirement to earn all mastery chests through your system of single champion mastery, so your initial rebuttal about willingness to put in the time seems ill founded.
It's possible that they should do more to reward single champion mastery (1), and it's possible that the hextech loot system should be made easier/more convenient (2), but those are separate discussions. If one established that both were things that should happen, then using one to solve the other could be a possibility, but it's far from a given, and there may be better individual solutions for each.
(1) = I wouldn't be adverse to a riot sourced mastery 'high score' list for each champion (and total across all champs), so players could climb through the ranks for their specific champion for example. Consistent S rank scorers earn more points than low mastery grade earners, so playing a lot and doing well would result in climbing faster than those just spamming the champion. There in turn could be some rewards for players standing on each of those leader boards.
(2) = I'd prefer if there was simply a 48 chest/season and 48*3 key fragment/season limit, rather than 4/month and 12/month respectively. It feels quite bad if I'm super busy one month (or on vacation or something) and don't cap out on my monthly fragments, and suddenly I'm forever going to be behind on fragments. Similarly, if I earn an S rank on a champion, it feels bad to not be rewarded just because I'd already earned one that week.