If we just use mathematics, then there is nothing to prevent granting a Cleric its 8th level feature at 2nd level.

After all, how does one compare the mathematical strength of a 2nd level Cleric Channel Divinity vs. the Warlock taking Agonizing Blast at 2nd level?

It's apples and oranges!

But both Clerics and Warlocks are fullcasters. Cleric has 1 more cantrip known than the Warlock, but as you've already mentioned number of cantrips don't matter much. Warlocks can get agonizing blast at 2nd level but Clerics get no boost to damage at 2nd level. At best Clerics get better weapon proficiencies depending on which Cleric Domain you take, but that's hardly a boost compared to adding mod damage to Eldritch Blast.

So why don't you grant the Cleric's 8th level feature to Clerics at 2nd level?

Throughout our conversation, you repeatedly call in mathematics, but never actually demonstrate it.

I focus on relative balance. You are right to say that the game is not perfectly balanced. It's not. What it is is relatively balanced. You can take a feature from an existing class and grant it to a homebrew class at a slightly higher or lower level than when it would come normally, so long as you can justify by putting it within the context of how the rest of the homebrew class functions.

But the only appeal I've seen for granting mod to cantrip damage at 2nd level for the Wizard is that the Warlock gets to add mod to Eldritch Blast as early as 2nd level, and that it makes sense relative to 3.5e mechanics.

Neither of which are compelling arguments.

Especially considering an Evocation wizard has to wait until 10th level before he ever gets to add mod damage to any spells.