I don't even think it's critical to play easy champions, although it's probably a good idea, and you should definitely avoid playing really high skill requirement champs.
Just play the same champ, every game, over and over and over. The only time you shouldn't play the same champ is if somebody else picks it. As Newt alluded to earlier, one of the mistakes people make a lot in Bronze is to keep trying to play different champions. If you are still learning generally how to be good at the game you don't also want to be learning the fine details of how to make your champion work. You don't learn to play a symphony at the same time that you're learning to play the piano.
i mean, the easy champions are often the ones with the high winrate. don't play thresh if you're bronze, play janna. don't play leblanc or azir or vayne or lee sin or whatever
if you're bronze you kind of suck at everything but mostly you suck at understanding the game in like a fundamental level. you probably know you have to ward but don't know when or where. your csing is bad. you might not know how to properly build in different situations, or when different champions' powerspikes happen, or what to do if you're losing or what to do after a fight. should you go for baron or inhibitor? are you stronger than the enemy team? etc.
mechanics are important but that's a non issue if you don't play demanding champions.
you can turn into a mechanical god later on, but for now a bronze player needs to start understanding the game, and so the less things blocking their way the better.
that's what i think at least, honestly i feel like the whole "i gotta carry 1v9" thing is bullshit when applied to regular people. if you're pro level sure, you can do that, but most people can't consistently carry games and are just held down by their teammates, most ppl just plain suck and don't even know how to carry
so yeah, imo ppl should play easy champions. or if they're gonna be like me and you find the most rewarding to play (and be trash at) champions like ori or leblanc or thresh or whatever, just don't complain if you can't climb :>
edit: not discrediting what you say btw, playing few champions is the way to go since you're never gonna be able to play at an acceptable level with them if you divide your time like that. again i personally enjoy playing a lot of different champions but i don't really care about solo q since i mostly play ranked 5s