Alternatively, I think of Java and Spring Boots as being incredibly valuable by letting companies that don't really have software as a core competency to make reasonably performant and structured applications.

Mediocrity will never not exist, and you unlock a lot of value by optimising for it.

Exactly. It allows to build boring stuff at a fraction of what a SV startup would burn using some fancier tech. And most problems in the world are boring stuff.