Not everyone needs to have the same experience. A common pattern is to make a responsive layout where CSS Grid isn't used on the smallest screens and that's the default. Many of the browsers that don't support CSS Grid are mobile ones anyway but even in a desktop browser they can get a not-ideal but usable layout.

Those 1 in 10 users don't provide enough revenue to warrant doubling our front end budget to support every polyfill, every workaround for every non-polyfillable thing, and the vast quantities of extra support they require.

If they do in your case, though, I understand.

True. Grid still has a long way to go. Flexbox works as a nice alternative in the meanwhile. However, the semantics of the flex is too confusing and cumbersome.