Why does youtube ultimately kills the reach of all old popular youtube channels?

Not necessarily. Viewer behavior changes, competition increases, and older subscribers become passive. When CTR and watch time decrease compared to newer channels in the same niche, the system will narrow its distribution because it prioritizes current performance over past achievements. Therefore, in my opinion, you should innovate and update content regularly, try trending topics, and optimize hooks according to current viewing behavior. You are retraining the system to understand that the channel is still relevant to market needs

If older youtuber keep posting constantly - this problem will come up only in case his content don't get same feedback from users as he had previously. With now activity on old youtube channels this is pretty common problem

the algorithm on youtube requires constant posting. Unfortunately, for most accounts when they are in the growth phase, they post new content almost daily. Of course they eventually burn out, and start slowing down and also since they have money they want to spend it and not spend too much time creating. So youtube lowers them when they post less, and picks up whoever is posting more