Considering that eldritch blast is the best cantrip in the game without any invocation support (as you said, high range, force damage, and the ability to multi-attack), I don't see how giving the other cantrips the same benefits via invocation could possibly be considered OP.
If you really want to help out other cantrips without removing Eldritch Blast, maybe make it so that Agonizing Blast gives +Cha to damage per die of cantrip damage. EB is still slightly better, but the other cantrips will give a better chance of big hits (at the cost of whiffing more often).
I think if Agonizing Blast applied to each die of damage, the damage cantrips that have useful side effects would be as good as Eldritch Blast. Fire Bolt would remain over-shadowed due to the damage type (about 80 core foes resist or are immune to
fire, versus 1 that is immune to
force, not to mention that spells like Absorb Elements have no effect on
force).
EB and invocations have been central to the Warlock, right from its 3.5ed Complete Arcane version where it was a ray (ranged touch) that did d6s. Hence it is right that commentators are concerned about removing it. Having played with Warlocks at the table for about two years now in 5e, EB spam is standing out as offering less fun than could be available with some adjustment. It seems to hedge the Warlocks in to using it (in part due to few spell slots, as [MENTION=20564]Blue[/MENTION] noted), and it also takes a pretty tightly optimised melee class to compete with it, forcing non-Warlock characters into a narrower design-space. (Warlocks have moved up in favoured classes, as measured by WotC, and I think that is in some part due to their sitting in a sweet spot in the 5e design space). Remembering at all times that here I'm talking only about the quality of the game mechanics.
The thing is, it feels like Eldritch Blast at the table is a bit better than it needs to be to remain viable. The combination of a target per damage die, force damage type, range, and per bolt bonuses, turns out to be really strong. Over time, the average damage from say Fire Bolt compared with Eldritch Blast sans Agonizing Blast is the same, albeit Fire Bolt is spikier (its damage comes in bigger lumps) and is more often resisted. However, the efficiency of being able to switch targets, and of course Agonizing Blast, make Eldritch Blast much killier.
I'm concerned that if all damaging cantrips are moved toward its level, then that could overshadow something else that my players in time will find they care about! What I'd love to see is reasonably often Fire Bolt being the better choice (over EB), without pressing too hard up against weapon-based ranged and melee. For that, the golden path could be ban or tone down EB, and buff the rest.
Say Eldritch Blast isn't touched, but the Invocations apply to only one damage die... doing so for
all damage dealing cantrips. It's interesting that Lance of Lethargy, written after Core was released and in play for awhile, already applies only once. Having seen them extensively in play at the table, I believe Agonizing and Repelling would be fine nerfed to once, and Spear could be simply triple the base range.