"They don't advertise or demonstrate how recent PCs are thousand times better than old ones."
It depends how old. The industry's real problem is that for most people their old PCs are good enough. There's nothing on the H/W side to require an update and, short of failure, they don't, therefore see any good reason to replace it.
Much the same thing applies on the S/W side as well. If MS had been able to come up with a must-have feature on W8 or W10 and made it a paid-for OS that might well not run on old hardware they might have sold a few extra boxes because of it. But they didn't. They came up with a couple of new versions which gained toxic reputations.
So, if the old box with its old OS is still doing what it was bought to do, why waste money.
And BTW, could you provide some quantified evidence for your "thousand times better".