Understanding where to put a bunch of class names is a lot easier than understanding CSS. Bootstrap isn't really about working around CSS, often it's about avoiding it. CSS Grids will likely just make Bootstrap easier to use, maintain and extend.
I personally stopped using Bootstrap years ago when I gained a good understanding of CSS. The most common features that Bootstrap provides (components and layouts) are not hard to implement yourself. And you'll avoid littering your markup with brittle combinations of class names and nested <div>s.
Such framework are abstraction + stylized components. They will continue to fill the same role.
I can tell my client's IT guy "just put a div with a class of .card and then a div with class .card-body in your WYSIWYG and you will have the correct display".
That the framework use floats or flex or css grid is irrelevant to the end poweruser.
A certain frsction of Hacker News commenters have declared Bootstrap useless and advocated just writing your own custom CSS from scratch since before Grid, and before Flexbox and before supporting IE 7 could safely be ignored so bear that in mind when evaluating the answers you receive.
I prefer making the template myself, but coding from scratch is not for everyone. Bootstrap is useful then depending on project, and what you can tolerate in terms of code bloat and bag of tricks bloat.
I pass the template I make along to the next project, and improve the template as I go. By using frameworks you're hand-balling the task of "making it better" to someone else. As for css grid, not using it yet due to my wanting to support older browsers, but I am now enjoying flexbox for page structure after ignoring it for a long time. Flexbox is a great set of tricks.
"By using frameworks you're hand-balling the task of "making it better" to someone else"
You sound a bit ambivalent about whether this is a good thing or not.
I would guess most hand-rolled alternatives don't have a long list of well documented browser bugs they couldn't work around and don't have half the workarounds for things they could handle. Instead they'll just silently break since they don't have the QA exposure across obscure platforms that Bootstrap does as a massively popular open source project:
I would guess Grid has just as many issues, and would guess that Bootstrap will adopt it at roughly the time it becomes supported by a wide range of browsers, and will have implemented and tested workarounds for any lingering issues with it.
Bootstrap seems to tickle the same "I could write Dropbox over a weekend" response that Hacker News commenters are famous for, just from those with a slightly different technical skill set.
Until Bootstrap adopts CSS Grid (v5?) I'd love to hear stories about folks using CSS Grid and Bootstrap components/typography/styling (i.e. everything other than Bootstrap's grid) together.
My opinion is that CSS bloat is way more harmful than bloat in any other language. I do indeed prefer to start from a blank state every time, or take a small template and eagerly cut everything that is not immediately useful out of it.
That said, I see the value of a CSS "library" for a large team that has to agree on both a template and terminology.
UI elements still need styling and normalizing. Grid is just one of many UI components in Bootstrap and similar frameworks. I imagine Bootstrap will put a layer of CSS class abstractions on top of CSS Grid to make it easier to work with, just as they did with flexbox.
I think Flexbox already removed the need for a CSS framework. Kind of the same situation as when improved DOM APIs (and browser compatibility) removed the need for jQuery.
Productivity. Bootstrap class names are still simpler than css and allow something to just stay within html. Also standardized themes and components allow for quick development compared to doing it all yourself.
