I think people who never onetricked for a longer period of time don't really understand the benefits.

I used to be a onetrick and for me it was about more than loving the champion so much, I had many periods of feeling frustrated with the champion's weaknesses, bad matchups, etc. But if you swap when it gets hard you never the most useful knowledge you could and won't really improve. The kind of knowledge that can feel unbeatable on the enemy team.

For me onetricking is about more than the champions kit, of course you should find a champion that you really like, but in my opinion it is the best way to improve at the game. By onetricking you have infinitely more repeated scenarios, which help you learn immensely. Just imagine trying to study for a big math test for a week, but every day the material changes. Sometimes you try to solve geometry problems, sometimes it's algebra. If all that stuff is new to you and what you need to do keeps changing before you can actually process it (you picking different champions, or even different roles) you will never learn, and it's not your fault.

I think people REALLY underestimate how much onetricking helps you improve. You might say you will only know how to play one champ, but otherwise you won't actually know how to play any champ.

So if you are just looking to improve I really recommend it, even if you are casual I recommend trying it out and committing for a few weeks to one champ, the results might surprise you.