Yes, mobile-first design has greatly accelerated the decline of usability in interfaces, but that decline also began before mobile-first really became a thing.
All I know is that usability keeps getting worse as time goes on. Where is the floor?
mIRC is very powerful, but most '90s, Windows power-users needed some explanation of how to use it, whereas Slack's design language makes it install-and-play for even basic users.
Mobile design isn't just a response to the limitations of its hardware; as technologies become more prevalent, intuitiveness to increasingly inexperienced users has to be prioritised over everything else and this is a major trend that dates right back to the beginning of the computer industry.
Usability is much better today than it was in the past; it's just no longer targeted at us.
All I know is that usability keeps getting worse as time goes on. Where is the floor?