I disagree with "practice makes perfect". You can keep practising something badly and you'll only get better at doing it badly. This can be seen everywhere. Second language speakers who have no interest in studying the language will continue to sound very non-native after decades of daily use. Musicians build bad habits that hold them back and take a long time to "unlearn". Programmers continue using the same old tools and techniques they learnt when they were students. You don't just get better by practising.
I don't buy your second language users example because I don't think that "use" necessarily equals "practice". It's usually easier to make major gains at the beginning of the learning journey, and things naturally plateau after a while, unless you put in more effort. Simply speaking the learned language at the level you have already reached will hardly improve your mastership (although I think it will minimally), you need to actually try to improve - which, to me, is the what practicing means.
Yes! That is why deliberate practice requires some mentors or guiding factor. It's really hard to invest in deliberate pratice and most of us overlook it! When practicing quality > quantity.
I gave up a lot of hobbies because I didn't have mentor or guidance factor to help me get past the wall. You can only learn upto a certain point on your own. After that, it requires quite a lot of attention and focus to not suck.
But the context being "something you enjoy", being passionate about something usually comes with self-motivation to improve and become better at something, and usually one finds a way to get better.
The stagnation you mentioned is usually with people who aren't that passionate about the things they are practicing, or just using it as a means to some other ends.
