Quite honestly, I think the Celestial Warlock is a subclass that should never have existed in the first place.

To my mind at least, the Warlock always seemed to be based on pacts with powerful beings that are, at best, neutral (with a great many being outright evil). Hence why most of their magic - especially their unique spells - has a dark or nefarious feel to it.

If you want a pact with a being in the upper planes, we've already got Clerics and Paladins.