While I agree with literally everything in this post 100% it also illustrates the trouble with the change being floated with the GM side of it a black box of unknown size shape & mass. It's entirely possible to remove crits from monsters and replace the d20 roll20 with a new mechanic that shifts the unknown and builds tension/excitement.. if done well.But there's more going on here in the game than just assigning damage to targets. There is tension and excitement that is created by The Unknown, and these things are a huge part (maybe even the biggest part) of playing a tabletop roleplaying game. Removing even a tiny bit of randomness from the game will remove an equal amount of that tension and suspense, and that would make the game less fun to me.
Lots of ttrpgs use a system where the gm has a point pool they can use to fuel various things, such a thing would allow the other side to expand the suspense and be meaningful. Lets say the new monster crit range is 20 to 20-[average party level-monster CR].
"APL"=average party level
crot generates a point for the pool
crot generates a point for the pool
- APL1: cr1/4 mooks - mooks crit on 19-20
- APL1: cr1 bbeg -bbeg crits on 20-20
- APL5: cr1 mooks - Mooks crit on 16-20
- APL5: cr4 bbeg - bbeg crits on a 19-20
- APL10: cr8 bbeg - bbeg crits on an 18-20
- APL10: cr3 mooks - mooks crit on 13-20
- APL15: cr12 bbeg - bbeg crits on 17-20
- APL15: cr4 mooks - Mooks crit on 9-20
- APL20: cr15 bbeg - bbeg crits 15-20
- APL20: cr4 mooks - Mooks crit on 4-20
That might sound good on papoer if players only fought high CR stuff with recharge abilities but it's a tiny fraction of monsters with those & mook grade monsters almost never have them of they have rather meaningless ones. This can be corrected with a wide array of generic powers the GM can pull from. These could be anything from quantum potions & spell scrolls chosen at the same time the GM invokes them to more specific maneuvers like automatically hitting/interrupting spellcasting/ dragging a PC X feet/etc.
The players know what's behind door number 3 marked "bbeg the dragon", but door number 1 & 2 are always a shrodinger's cat even if there are limits like "the GM ability can only use this ability once per combat/round" or "the cost to use this ability doubles each time used per round" & "choosing the same potion/spell more than N times per combat is not possible"
Of course if 5e is anything to go by I wouldn't be at all surprised if the other side was "when the GM rolls a 20 on a d20 test with a monster that has a recover on 5-6 ability it automatically recovers at the end of the turn even if the d6 roll was not 5-6"