You are not entirely wrong but I would start worrying even more when these centrlized entities start having over 30% of the max supply of Bitcoin, like OP just said, it's 5% for now, out of 100% and we are worried? It's clearly not the time to be worried.
A year ago, before the ETF was approved, that figure was even lower than 1%. So while 5% is not a big number, it is a sign that it will continue to increase in the future. Even that will happen much faster as more centralized institutions get involved in accumulating bitcoin and probably won't stop at 30% like you think, even 50% or more. So that's really something to be concerned about and worried about from now on.
But as you said, unfortunately there is nothing anyone can do, nothing can stop them.