Interesting thoughts indeed ?
I have not yet actually used CSS Grid in a real life project yet, because some mobile browsers have just recently started to support them (https://caniuse.com/?search=grid – Date Relative tab) but probably sometime next year it will be"almost safe" to use it even without a polyfill.
I think CSS Grid is for the "main layout" of a page and not for the layout of individual components. Sure, CSS Grid could be used for individual components as well (why not?) but it looks like it would be an overkill for that purpose. However, flexbox is still great for individual components. The grid systems in CSS frameworks are pretty useful, but in the light of CSS Grid, in the future I will probably only use them for individual components, and CSS Grids for the layout of the page.
Being able to freely and "responsively" move blocks around the page based on viewport size is the real power of CSS Grid, I think:
https://www.youtube.com/watch?v=TrLN2YId-5M
https://caniuse.com/?search=grid-template-areas
This "rearranging-freely power " is why ditching the classic layout grid systems for page layouts is the future ? Not to mention that the various framework grid systems require various levels of wrappers to support their specific grids, while the CSS Grid is standardized, of course.