You don't really need to break into the multi hit scenario, where the math gets really painful.
Crit rate is independent of hit rate. No matter the AC of the target, you crit 5% of the time.
So, assuming the 17th level case, the added bonus for crit is either 5% of 4dX or 5% of 1dX repeated 4 times, which sums up to the same average. The distribution is different, of course, but considering that crits and especially multi-crits are edge cases anyway, that really isn't very applicable.
Yes, but you have to control for the fact that you might hit just once (and thus that attack can crit 5% of the time), or you might hit twice (and thus either or both could crit), or you might hit three times (and get 1/2/3 crits), or four (etc.)
A one-hit cantrip like standard
fire bolt is all or nothing; either the whole thing crits or it doesn't. The math is very simple. A multi-hit cantrip like
eldritch blast, you have to account for all the possible cases. The raw probabilities I used above (for "just treat crits as regular hits, figure out the bonus damage later") were crunched for me by a binomial distribution calculator; I used those as conditional probabilities above.
It
is an average DPR bonus to be able to crit on each individual die, not just the whole kit and kaboodle. You sacrifice a "high" (5%) chance to do ~22 bonus damage, but you gain a very high chance of doing at least
one crit. Consistency does, in fact, shift the center of the distribution up a little bit.
Until I make further changes (which I assuredly will), Warlocks still have the AB benefit. 4d10+20 is still double most classes 4d10. The main benefit of my proposed change without other changes are subclasses like Draconic Sorcerer, where their cantrip damage for their chosen element will go from 4dX+5 to 4dX+20.
Sure, I'm not saying AB is going away. Also, I don't actually think Dragon Sorcerer is a problem here; Elemental Affinity says "to one damage roll of that spell." If you're careful with your house-rule wording, it wouldn't apply. E.g., call it "Extended Cantrip," with text saying something like, "Starting at 5th level, when you cast a cantrip that requires an attack roll or forces a target to make a save, you may apply its effects twice instead of once for a single casting of that cantrip." The crit benefit remains, of course, but this phrasing would mean you may make two/three/four attack rolls with
fire bolt, but you can only apply Elemental Affinity to
one of the damage rolls triggered by hitting.
I'm just saying, there are three benefits to
eldritch blast: typing, multi-hit, and Agonizing Blast. Typing is a small but nice benefit. AB is a large and nice benefit. Multi-hit is a medium and nice benefit. Warlock, as a class, is one of the ones that needs love, not being relatively nerfed by having its distinctive features weakened by comparison.