sure, players improve, some more, some less, but they don‘t transform magically into something different. murray improved his forehand but it was still a weakness, so is tsongas backhand, raonicsbackhand, paires forehand, zverevs forehand, tsitsipas backhand etc. and they always will be a weskness at that level. it is also relevant how bad or good you are at a certain skill. you can argue that the lower the base value, the more room for improvement, but that also means that you have to improve a lot to turn a weakness to a level on which you can rely on in important matches at the highest level. and if you are not comfortable enough, you won‘t use it in tight moments (trying to finish the point at the net instead of trying to hit through your opponent endlessly at the baseline).
furthermore sinners problem is his bad touch at the net and in cat and mouse points, and how to improve that ?
i like sinner by the way, but he is who he is.
p.s. djokovic had a good serve, then it got bad after the racquet switch, and now it isn‘t that great like some are saying, so his improvement wasn‘t that big.