Whenever I see default Bootstrap styles out in the wild though, I get pretty disgusted.
Never trust a company that uses default Bootstrap styles in their production apps. It just shouts "I don't care" or "I don't know any better."
I've seen a bunch of stuff submitted here to HN that was nothing more than a landing page with a sign up form. I've rarely (if not never) signed up.
However, give me a page with a working proof-of-concept, I'll likely sign up regardless if it's built on default Bootstrap styling.
Reminds me of the days when people's biggest complaints about Reddit was that it looked bad.
Sounds like a pretty positive indicator to me.
But a lot of the time it really means "Let frameworks do everything for you." Don't reinvent the wheel, but feel free to reimplement subsets of the wheel that aren't all too tailored to your purpose. In fact, if you're a programmer, you probably do that all the time.
Particularly when it comes to web design, there is no specific "wheel" to speak of. I'm all for using Bootstrap... as a framework, not as a template you copy and paste.
I can make exceptions for local web applications (like router interface panels) and small, one-trick apps, though.
I'm happy to let a framework do everything for me - assuming the 'everything' you speak of is done well.
Implicit in your statement is an assumption that this will be bad - I'm not disagreeing that there are downsides, but please don't hide these inside implied reasons. Bring them out and let's debate them explicitly.
For many products however, these days an "interesting" interface doesn't sell it unless you are pinterest or something. 99% of the time, users are at your page/app for some level of function. Rarely is the function essentially the form.
Whenever a web app doesn't look different, complaints flood in.
Consistent styling on disparate web pages is a good thing, in my optinion.
The number of times I want to be 'surprised and delighted' by an interface can be counted on the fingers of one hand. Most of the time I'm trying to achieve something other than appreciate the genius of the designer involved.
For example - I think many of the templates here: https://wrapbootstrap.com/ are rather lovely. Do they elicit your disgust?
Reductio ad absurdum...