I will say this: Damage is a constant.
As in a character that can deal an ungodly amount of damage per round consistently is going to be a big boon in any D&D game (barring the few where combat is extremely rare; I haven't seen one of those yet).
Spellcasting and magic is less of a constant. In one person's D&D game a warlock with prodigy, actor (+ relevant skills) and mask of many faces is utterly game breaking. In another person's game it could be entirely useless.
That's the problem. The original claim was "The champion is still unlikely to be as good in both melee and ranged combat at the same time as the hexblade. They don't need any slots at all for this.". That last bit of your post obviously falls well outside his claim. If he wants to talk about noncombat elements like those it's fine to do so but doing that involves actual discussion. Vaguely alluding to spells solving nonspecific edge cases the other characters without a (generally) unnamed spell is not doing that. Listing a bunch of noncombat spells in isolation void of any context or anything likewise is not discussion.
I agree that the warlock changeling you describe could be very powerful in a highly specific niche of campaign.... However... you could avoid the need to be a warlock and just play a changeling & get some shiftweave. I also think we can both agree that such a campaign is going to be quite unusual & even moreso if that already unusual campaign is also one where combat is rare.