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.
you are right, and that is a great solution if you don't care about offering the actual experience to all users.
If the original problem you need to solve is "using css to style a website so all users have the expected experience" then you need more than 5 lines of css.
"The" actual experience is a myth, all users are not going to have the same experience no matter what, that's one of the points behind responsive design. What I was presenting was an alternative to ignoring CSS Grid because someone thinks it doesn't have sufficient browser adoption.
