The language is sort of settled. The browser runs JS, so for most projects, it's the right choice.
> If your large, complex, react app is slow - what is easier? Fixing the data architecture or adding a useMemo?
It might be easy to add useMemo, but it doesn't work. People using a tool for the wrong thing because they don't understand it isn't a problem with the tool. It's unfortunate that a tool with a steep learning curve is the "default" for many devs now, but again that isn't a problem with the tool. I don't have any guardrails keeping me from using my saw as a hammer, it just doesn't work.
> because something about language/framework/api makes doing the proper thing hard or unclear.
It's unclear because it was built on functional programming concepts and most devs aren't taught that way. If you are familiar with those concepts, the whole thing is quite simple.