... apart from the fact that you will never ever take damage from failing a casting threshold or even empowering it, it sounds like a neat idea.
but the lack of consequences makes magic just another tool and not a wavering and dangerous thing that should not be tempered with.
thus: "thanks but no thanks" for me.
Indeed, it would mean that you'd be less likely to take damage, but rolling a 1 is still rolling a 1, right? However it might also mean that wizards more consistently performing some sort of useful magical action, and mitigate some unfortunate die rolling that might otherwise ruin all your plans. Unclear until I can actually try it.
Personally I've never really liked taking damage from low spellcasting rolls as a rule anyway. It feels like it needlessly discourages trying to cast the more difficult spells. I've also been considering using a homebrew fumble table for spells instead.
while we're at it: has anyone tried to substitute 1d20 with 2d10 and leave the crit out of the game?
I don't typically use crit in my games. People I've played with generally agree it's a little extra. As far as as replacing a d20 with 2d10, I have never tried it. What I know about probability, with 2d10 you'd mostly be rolling between 7-14, and it would no longer be possible to roll a 1. It would certainly make things less swingy, but might also make it very difficult to damage high armor characters, for example.