The decline in usability ("enshittification") is due to a readily evident change in incentives behind the software.

You're no longer selling a shrink-wrapped application in a competitive market, where you compete on usability.

You're giving it away, while figuring out how to extract value out of the user in other ways.

Online services that have captive users (e.g. the one and only government site you have to use for some service) don't have any incentive to improve their user experience. But that is not new. There was no previous version of that which had a better user experience; the previous version was an obstinate bureaucracy driven by paper pushing in the backrop of rigid protocols. What you see is just the computerization of that.