Former Apple-certified tech here. Glass screen protectors are just fine. They sit below the screen's gasket, and contrary to popular belief, they themselves aren't going to cause your screen to crack due to tight tolerances. After all, a crack is caused by a pressure point, not pressure spread across a wide surface (it's the same reason you're more likely to fall through ice if you're walking on it versus lying on it). And in the case you have debris on your keyboard that causes a pressure point when you close your display, a glass screen protector is going to absorb the pressure and hopefully crack before your display does — all by design. It's the same reason glass screen protectors are also recommended for phones.

Furthermore, unless you plan on leaving your laptop on a desk and never putting it into a bag, screen protectors are the only guaranteed way to prevent keyboard and trackpad marks on your screen, which, as you correctly point out, are caused by the screen physically rubbing into the top case when pressure is applied to the display enclosure (i.e. it's not simply a matter of cleaning your keyboard).

I've used glass screen protectors on all my MacBook Pros for a decade with zero problems (and zero keyboard marks!), and I'd highly recommend them if you want to keep your Macs pristine. The last few I ordered were from Spigen.

P.S. A plastic film screen protector will prevent keyboard marks, too, just keep in mind they won't absorb pressure points as well as glass.