I agree the problem is crits not feeling special, but truth is spellcasters attacks almost all scale with level.
so its not 1d10 versus 1d10 spell vs weapon, after 5 its usually 2+d10 vs 1d10 on weapons. basically spell attacks are designed to get more damage dice. The 'critical' of spells is rolling well on many dice. In the current game crits strongly favor magical abilities, the meta for effective crits is to build large dice pools.
they could give spells a single extra die on crit, but would it even be noticeable as a crit? with say 4d6 versus 5d6 the luckyness of the rolls will tend to outweigh the criticalness.
the problem I think is the psychology, will rolling a 20 still feel great if its fairly weak? This is probably why they are making crits give 'inspiration' The big deal becomes the ability to force advantage or disadvantage, which casters can use to weaken saves on most their spells.
but will a delayed use of inspiration create a great enough feeling on rolling a crit?
Yes, Firebolt and such scale with level, but so do class abilities like Sneak Attack. When you consider that, 2d10 on a CRIT is nothing compared to a Rogue CRIT with Sneak Attack 3d6. Think about it. Rogue wields a Rapier. 1d8 +3d6 with Sneak Attack. CRIT. 2d8+6d6. Yeah. Slightly more powerful than 2d10.
That said, it is relative to a higher power wizard spell or a cleric Guiding Bolt, etc. But again, that's the point. Mages not able to crit nerfs them. And remember, most powerful wizard spells don't make you roll to hit. They make targets roll save throws for half damage, so Crits aren't possible for spells like lightning bolt or fireball. They're only possible for spells like Firebolt where you roll to hit.
