If i knew the answer i would be already 2100 many years ago...
IMO the answer is not simple and isn't unique. Every player is unique and there are many dimensions in a TT player. Some players may not be able to play a BH loop at all, but if they have a very good footwork and can play all-FH its ok. Or if they can't really attack but can retrieve everything in defence... etc...
I think there is no fundamental difference between 1900 to 2100 or 2100 to 2300 or 1700 to 1900.
You have to improve your game overall until everything clicks and you get more wins.
sometimes you have some real improvement in part of your game but weaknesses which are too obvious.
if weaknesses can be fixed then suddenly there is a "jump" in level, because those weaknesses were costing too many points.
There are many many details to work on. foot placement, balance, footwork , balance , grip form , timing etc...
i can't understand anyone can say i can't get better. There are tens of things we could improve on.
This year I took the new initiative of taking some notes about opponents in tournaments and writing down important advice so i don't forget about it.
I keep on training with coaches 1 on 1. Been doing so for more than 10 years. Improvement in RESULTS has been slow. But i believe I'm on the right track. With my current coach (GC), what i think is EXCEPTIONAL compared to my previous coaches is that he's been with me to tournaments and seeing me play live. So he could see the real level i'm playing at, the issues I need to fix, and we're working on that.
I think one cannot improve if one doesn't like training and practicing. because results don't come easily and quickly. Tony is right, its important to train with people of (much) higher level else you can't get there. But equally important to play with people of same or lower level, and of many many styles. In tournaments, i'm actually more in difficulty with those slow knuckle balls or serves or LP that i don't play enough in practice.
I think working on fundamentals, and doing footwork or multiball is never a waste of time. nor are serve drills, or 3rd ball drills etc. the most important is having short term and long term goals and not waste your time at the table.
My "long term" goal is to perform well at Asian Veteran tournament in October at Yokohama. I could lose 100 matches before that, a few good wins there will be worth it.