Okay, to help gather my thoughts I consulted with my brother who has a degree in design and who has been a UX Designer for the best part of decade.

There's a couple of issues I have with your argument.

The first is that when I made the distinction between web design and web development you said that's not the point. But I think that is the point. It's right there in big writing on the Bootstrap homepage (emphasis mine):

Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web.

Front-end frameworks are not built as design tools for web or UX designers. They are built as development tools to help developers. If people's designs are becoming hamstrung by frameworks, then that's an issue with the people themselves, not the tool.

Now there's an argument that could be made that developers who otherwise have no idea about design take these tools and create functionally solid but generically designed products. There's certainly some truth in that. But it only serves to reiterate the point that it's about how the tool is used that matters. And as an aside, I'd much prefer to visit these stable but generic sites vs. stable sites that lack any semblance of design or consistency.

Another point that cements this is that for every indetikit website created that has their foundations in Bootstrap, there's plenty of amazingly designed ones too. Ones with creativity and imagination. If your point about these opinionated frameworks being analogous with someone instructing you to compose a song using only a specific chord progression is to be upheld, surely this wouldn't have been possible?

The second issue is that i'm unsure what you're constituting as an opinionated framework? Bootstrap is the primary name cropping up in this discussions. But like I stated before, you can heavily customise the implementation of Bootstrap to fit your specific purposes. Seriously, look at how many of the opinionated elements of it you can chose to completely remove. You could just have the grid if you wanted, with no CSS, pre-made components, or JavaScript at all. What's more, you can change the default values of that grid too. This seems to be the case with a lot of mainstream frameworks.

Again, we return to the central point that these frameworks do no intrinsically promote bad design and taste. Rather, it is bad and/or lazy designers that promote bad design and taste. No designer worth their salt is looking at Bootstrap's default guidelines as a reference when they're designing a site. Sure, there's chance there's subconscious influence. But part of being a good practitioner in any field is being aware of the invisible forces and constraints that are pulling at you, and in turn working with and around them to create the best final product.