I want to point out that not studying opening theory and not studying at all are two very different things.
I think you can get to 1500-2000 probably without studying opening theory, but without studying at all it will be way more difficult than with studying endgames, tactics, opening principles, middlegame principles and endgame principles etc.
I think serious opening theory study will only be needed at something like 1800-2000 rating, but I have studied some opening theory as a part of my study plan, since I don’t think it hurts to spend a small portion of my study time on openings as long as I do not neglect the other aspects of the game.
What I have noticed is, if I play a lot of blitz, my game will eventually just start deteriorating and I lose the plot and have to take a break to be able to play again at all.
After a lot of blitz I went back to rapid and tilted my rating from 1500+ to closer to 1400. I will now only focus on studying and longer games and try to play as much classical chess OTB as possible. Getting a little offtopic, but I guess my point is, speed chess will probably only do harm at some point.
Just curious about your opinion and experiences. I am only approaching 1000 ELO and up till that point I haven't really been reading and learning any openings, but I wonder how far did you go yourself before not knowing some theory, lines, openings etc - in other words, studying to advance - would just prevent you from advancing further.
Or maybe you have totally different approach to this. Would love to hear some opinions :-)