Granted, not every component on every surface will need this amount of scrutiny. But I'm usually the outlier developer warning teammates that design is not a solved problem. Granted, there's a huge difference between an existing app and its evolution and throwing a nextjs landing page up in search of any life.
"make things pretty" would be a graphic designer or artist. Are you saying the entire arm of Product design is a made up value?
So you can take bootstrap (or even raw html) and create something useful. Then you make it nice, not the other way around.
You would have to be a big outlier to feel the need to create a custom widget. Most widgets have been defined since decades.
But i don't think that's what tailwind and bootstrap are doing. But people very much use these tools to "solve design".
The layouts, widgets, and primitives in these tools are not primitives. I can't deny they get tons of people very far very fast. But my main disagreement is that all of this isn't design and it's not what designers do. You touched on what i agree with: UX flows, diagrams, stories, journeys, personas, etc, these all need to be designed and connected in reality using various primitives for the medium.
Then you slap a cohesive paint job on it, interaction elements, tone and terminology and yes, there is that element of design too.