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.