How often can you use Bootstrap components unaltered? I'm either working on things where aesthetics are important enough that I'd have to write custom CSS for every component, or where aesthetics are so unimportant that stylistic coherence is irrelevant.
When I use things like Bootstrap it's as a scaffolding in the early stages of development before I write enough of my own CSS. Eventually then I've restyled all the Bootstrap components and I only keep it included to avoid spending 10 minutes re-implementing modals in JS.
Isn't that the whole point of Bootstrap? It "bootstraps" your design to get you to a minimum coherent design with the least amount of fuss, and then allows (some would say forces) you to build on top it to customise as you see fit.
Overriding Bootstrap variables gets you 80% there. You can do a lot with changing padding, colors and fonts. I was very sceptical of Bootstrap from a design perspective but changed my mind after some deep diving into customization.
When I use things like Bootstrap it's as a scaffolding in the early stages of development before I write enough of my own CSS. Eventually then I've restyled all the Bootstrap components and I only keep it included to avoid spending 10 minutes re-implementing modals in JS.