I think that is a poor criteria to drop spells (which I have changed my mind on as the thread has continued). Ultimately how often a spell is used isn't the primary metric, but whether that spell serves an interesting function.
The real question is: Is the spell hitting a niche we think makes sense for a spellcaster to have? For example, is the idea of a spellcaster disguising a large amount of terrain to look like something else a niche that we think is cool for a spellcaster to do?
If the answer is yes, than its about ensuring the mechanics fit the flavor. People have noted that the area of hallucinatory terrain severely impacts its use. If that's the case then it might make sense to bump up the area (and maybe increase the spell level if a large area illusion is just too good for lower levels).
For other spells, its really about duplication. Damage spells are the classic ones here, there are a LOT of damage spells in the game. Are all of them REALLY needed....probably not. A few flavors certainly, but you don't need 20 different damage spells to do the job, and so you could put several of them on the chopping block without really impacting the game.