Since CSS grids are mostly used for the global page layout, the ideal use case is using it to design complex pages that would be very annoying to make with other techniques, and on older browsers fall back to a much simpler design with flexbox or floats.

Since those fallbacks only require a handful of lines of CSS, it only takes a few minutes to get the compatibility to almost-perfect, and you can get a better design in front of most of your users very quickly. To me at least it's a solid mix of design freedom and development speed.

If you have a good reason for complex pages your solution is only viable once the share of old browsers has dropped below, say, 5% to 10%, depending on your niche.

What's actually going to happen is that most people will layer JavaScript frameworks on top of HTML and then we're going to complain about the huge performance and memory overhead of JavaScript heavy websites.