Adult starters can improve but only a little, there's a hard limit, like it or not, you can see a lot of examples here and even among titled players. Has nothing to do with me. Purely physiological limitation.
Just to remind you what you said since you seem intent on moving the goal posts. Next to no one is going to be hard stuck at 200 elo mate. Practice dedication and a willingness to look inwards for mistakes instead of blaming it on something you can't change works wonders.
Create a study with a batch of over 5000 Adults (example) on chess.com, track their improvements and playing times on rapid, then cross reference it with teens and even kids and their improvements from short term to long term. Start with a semi low ELO (like 500 for example), and they have to be playing at least 4 days out of the week with more than 4 matches a day. You can also take into account daily rapid and blitz games. This is a actual solution that will show the proof you have of this issue. If you can even manage to do this, I'll rescind my argument entirely.