I think one big issue is that companies got massive. Apple's revenue, for instance, would make them the 40th largest country in the world by GDP - larger than Portugal, Hungary, Greece, and the overwhelming majority of countries in existence. And it seems to be essentially a rule that as companies reach a certain threshold of size, the quality of what they produce begins to trend downward until they're eventually replaced by an upstart who then starts their trek towards reaching the threshold of fail.

But in modern political and economic times where number must always go up, too big to fail is a thing and anti-trust enforcement isn't (to say nothing of the FTC mostly just ¯\_(ツ)_/¯ with regards to basically any merger/acquisition of big tech), the current batch of companies just keeps growing and growing instead of being naturally replaced. To say nothing of the fact that a lot of startup culture now sees being acquired as the endgame, rather than even dreaming of competing against these monstrosities.