Glass absorbs light, but if you're talking about
visibly dimming, a good protector won't. (The amount of light you get through the protector will be a measurable amount less than without the protector is you have sensitive enough instruments.)
Remember, though - a soft protector scratches more easily but breaks
less easily. The harder the protector is (and some of the really good ones are about 7 on the Mhos scale, which is pretty hard), the more easily it breaks, so dropping the phone on its back or a corner can break a good tempered glass protector. All tempered glass does is protect the screen against scratching. (And carbide - which is in the air all around us) has a hardness of 9, making it cut through tempered glass like a hot knife through water. [Butter is hard by comparison.] So if you don't wet the microfiber cloth you're cleaning the screen with, all you're doing is smearing the finger grease and - if there are any micro carbide particles on the glass - scratching it.)
I prefer a good plastic protector (you can get plastic ones very cheaply) and when they scratch (I've used the same one for 3 years), it's less than $2 to take another one out of the pack and put it on (after cleaning the screen, of course).
BTW, as far as these "9H" screens go, see
True Hardness of A Tempered Glass Screen Protector. They aren't 9 hardness. It's like Apple - you're paying for some print on the package.