Yea, in my own groups, counterspell is cast and hope for the best, we don't have spell ID rules. So hitting 4th and 5th level spells (a fairly broad swath of spells) is a noticeable boon.The auto-upcast helps if and only if the spell you're countering is 4th or 5th level, otherwise it's inefficient; though if you don't get a way of identifying the spell before countering it, then the other casters have to guess what slot level to use anyway. This is one of those things that depends on the DM; there are suggested rules for ID-ing spells in XGtE, but I don't know how many people use them.
Sure. Having a reserve of spells to pull out when the situation demands it is obviously the single biggest benefit to playing a caster. But it's not like having a counterspell available is an either/or choice. If it's on your spell list, you should probably have it prepped or known. Having 2 casters in the party with counterspell also lets you shutdown the enemy mage counterspelling an initial counterspell.The bigger part of the argument against warlock being the primary counterspeller for me is that they are set up to spread their spells evenly throughout the day, which to me means about one spell per encounter for most of their career. Counterspell is one of those spells where most encounters you don't need it, but when you do need it, you might need it a bunch of rounds in a row. Regular-slot casters have the ability to use their spells unevenly if the situation demands it, whereas warlocks can't.
Sure. But again, I'm not arguing that the warlock should have it instead ofthe wizard or sorcerer (or bard with Magical Secrets), I'm arguing they should both have it but the warlock should be the first choice to cast it.At 5th and 6th level the warlock has the advantage, since regular casters only have 2 or 3 3rd level slots anyway. But by tier 3, they get seven or eight slots between 3rd and 5th level, and so blowing 4 of those in a single mage encounter is not unreasonable.