What has happened that people accept lower and lower quality?

Same thing that's driven quality off a cliff in every other product category: enshittification. When you can buy up your competitors, sue anyone who tries to improve or repair your product, and replace any employees who try to stick up for quality with cheap offshore labor and/or AI, there's nothing to stop you from making your product shittier and shittier.

Which invokes the question: What has happened that people accept lower and lower quality? Is new software nowadays still really useful?

Users are mostly not programmers or anything related and will only notice bad quality, when it has piled up for a longish time, because they only observe the consequences of bad decisions and practices and don't experience what bad code impacts first: the work on the code itself and the ability to add new functions or correct mistakes made.

Coding isn't a problem.

What does solved even mean here? Might as well say human beings are not solved.

My pinkys are also malformed from years of chronic shift key and mouse finger dragging. I know how it be