A concerning downside to hex is concentration and the ease in which a warlocks concentration can be broken. The higher level encounters you face the more likely your concentration is to get broken. Not to mention the warlock has some additional very good buffs and control spells that require concentration later which hex competes with.
For levels 1-10 hellish rebuke deals nearly identical damage to hex in typical encounters (if not better), does it's massive amount of damage earlier in the fight (damage earlier is always better than damage later) and is guaranteed unlike hex which can be ended by failing concentration saves the turn after you cast it or any turn after that.
The damage caused by hex with 4 attacks against a 15 ac foe at level 4 is 8.4 damage.
The damage caused by a level 2 hellish rebuke against a +2 dex save foe at level 4 is 12.7875 damage.
Even at Hex's sweet spot of level 5 and 6.
The damage caused by hex with 8 attacks against a 15 ac foe at level 5 is 18.2
The damage caused by a level 3 hellish rebuke against a +2 dex save enemy is 17.6
Please remember that I'm not arguing about Hex being good or bad, only about Eldritch Blast with the Agonizing invocation. As you say, at higher levels there are better casts than Hex. I also agree that Hex is not reliably "on", contrary to some analysis I've seen on these boards. In play, it's often down for some number of rounds. In an adventuring party, using the spell and concentration slot to grant GWM or SS weapon-attacker persistent advantage probably out-damages either.
I'm all for letting it apply to other cantrips. But why limit it for EB? The ranger already out damages you with no feats all the way until you hit level 11. Why further that gap. It's likely the battlemaster fighter outdamages you as well (even without feats). And both can definitely outdamage you with feats.
What melee character are you trying to compare the warlock to that you think doesn't get either the 20-30% higher damage than him or strong enough defensive abilities to compensate for the damage difference?
That's a fair question, and as you know one has to make some decisions when forming scenarios. The characters that I use as yardsticks are created along strong lines, without going far from what I see people creating at my table. For example -
Ranger Hunter with Archery style, longbow, Sharpshooter feat
Fighter Battlemaster with one-handed weapon and shield, Shieldmaster feat
Barbarian Totem (Bear), Reckless Attacking with greatsword or greataxe, GWM feat
The contexts in which I evaluate them are based on the game as I see it played. Roughly three encounters per long rest, about 5 rounds long each, facing mostly "hard" foes. That helps suggest the ACs, saves, and hits-back that are likely to arise, and the probable "up-time" for strategies based on resources, positioning, action-economy and ability to stay in the fight. Taken together, I'm not looking at some of the more optimistic builds like Precision/GWM/Battlemaster, or cheesier ones like CEx/SS/Hand-crossbow Archer.
It's a pretty big deal even at tier 2. You'll have somewhere between 1/3 and 1/2 the time that you hit with both attacks on a turn. That's often enough to matter.
Sure, what melee character are you worried about outdamaging with the warlocks agonizing blast?
So the GWM Barbarian who is all-in on a toe-to-toe attack is for me the yardstick for repeatable, single-target damage in D&D. Discarding defences to wield the biggest weapon that they can find! For me, it'd be a bit underwhelming if that
didn't define the envelope. On average over an adventuring day (i.e sustained DPS) I put them at about 10/round in tier 1, 20-30/round in tier 2, 30-40/round in tier 3. Those aren't the biggest numbers they can achieve, but over time it matches quite well what I see happening in play.
The 1HS Battlemaster has a solid AC and gets persistent advantage through Shieldmaster: I think they look like a good yardstick for a conservative melee strategy. I put them on average over an adventuring day at about 10/round in tier 1, about 20/round in tier 2, and about 30/round in tier 3.
In both cases, I expect the melee attacks to out-damage ranged. This means I am looking for a 120' ranged attack, like Eldritch Blast, to do say 7 in tier 1, 14-21 in tier 2, 21-28 in tier 3. Taking into account the possible on-hit crowd-control effects, and following what I've seen happen over numerous sessions of play, I'm drawn to the bottom end of this range. I find that with Once-per-turn it deals nearly exactly that. Without Once-per-turn, i.e. as RAW, it deals at the top. In tier 1 there is literally no difference. At tier 2, Warlock might roll for 30 beams a day, missing about 8 of them; the 22 hits distributed over 15 turns. Therefore losing on average about 2 damage a round. At tier 3, Warlock might roll for 45 beams a day, missing about 9 of them, losing a much more significant 7.5 damage per round.
But this is not what happens in play. In play, Warlock at tier 3 has 3 high-quality casts that they'll usually recover right after the fight. They'll drop Synaptic Static on some group. Greater Invis. their GWM Barbarian (persistent advantage does crazy things to their damage dealing.) D.Door out of trouble. I think they'll usually miss at least two rounds a combat. In that sense, your Rebuke strategy is actually kind of great! So that is really 27 beams, missing about 6 of them, or again about 2 damage a round.
It even seems possible that Agonizing was just a wording bug that they decided to go with, when I look at how hedged most other damage-adds for spells are (i.e. to once per time damage is dealt). Agonizing blasts are d10+stat attacks, that scale for extra attacks at the same rate that 1H fighters scale their d8+stat attacks, and then have 120' range, force damage type, and on-hit c-c effects to boot. Superiority dice keep the Battlemaster about even, and can provide c-c. So for me the direct contrast could be simplified down to what 120' range and force damage type are worth in play? I think they're worth something: what seems reasonable?