Given CSS Grid, what role do frameworks such as Bootstrap play?
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.
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.
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.
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:
https://getbootstrap.com/docs/4.0/browser-bugs/
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.
That said, I see the value of a CSS "library" for a large team that has to agree on both a template and terminology.