There is an extreme false equivalency comparing champions to college majors. VERY EXTREME.
How many things will people do in their lifetime, between learning how to talk, read, drive, cook, work at various jobs, count, do math from simple arithmetic to calculus, play video games of various genres, remember people, places, things, identify sights, sounds, smells and textures, and so on? The way you describe it, you act like trying to learn ONE champion is like trying to learn the quantum mechanics of the universe, and that you have to neckbeard yourself to death to even get marginally good at one.
That isn't how it works. The human brain is capable of near infinity, but people squander it on paranoia, fear, blood, brussels sprouts, and self-contradictory bullshit, not to mention that we all have different starting lines. Some people are born geniuses, others will struggle to count to four until their dying days. Hell, in America a lot of teens don't even graduate high school.
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https://i.imgur.com/28lw367.png
I don't neckbeard myself over this game, and I have become more proficient with more champions than most players ever will, especially by your standards, AND I play other video games, AND I write, AND draw, AND currently learning another language, AND working on game design, AND keeping up with American politics, AND managing a tabletop game on the weekends! And when Kled comes out, I'll pick him up when I have the opportunity, figure him out and decide what place he has in my worldview.
I could exclusive Braum or Lulu until the heat death of the universe, but I probably won't learn anything new that my current crusade to Mastery 5 and beyond didn't teach me already. You don't just bumblefuck your way to the Mastery level (240+, as of now) I am at right now. I certainly won't learn anything new if I end up playing against the same ten champions, basically falling back on the same tactics and not learning how to deal with new menaces.
Want to know the best part? I didn't really play Lulu for half a year while I was building my Mastery with other champions. Then when Masteries 6 and 7 came out I went back to her just briefly enough to go on a warpath and get her to Mastery 7 in less than a few days, DURING MY FUCKING SPARE TIME.
THAT is the trap that everyone, including the professionals themselves, have fallen into. Everyone learns at different paces, and people have bought into the idea that the only way you can get even marginally proficient at anything is to go full neckbeard and exclude and basically dump stat literally everything else in your life in favor of that one thing. That isn't how it works. League is not motherfucking quantum physics!
People aren't advancing not because they aren't focusing hard enough on that one thing. It's because they aren't focusing on enough things, and this overspecialization has spread out from entire divisions to entire populations. Most games I play, you have nothing but ADC and assassin mains who will collectively abandon warding, claiming objectives, pushing and defending for teamfights and pentakills, even when they have absolutely no right to try and focus on those things, excluding even the game itself for some illusory zenith of mastery that people have mistaken for reality.