Keep in mind that, if Iwobi was actually good, he should be well past the six players from Brighton in the top twelve as the primary focus of an attack. Tarkowski nearly doubled up the next player behind him for blocks this season. (Tarkowski having the opportunity to make two blocks per match, on average, is its own problem unrelated to his quality.)
Truth, I think, is closer to where you're coming down on this one. Iwobi isn't 'diabolical', to use Mike's word, but he isn't particularly good. Replacing him at this juncture is, as you say, probably unwise. For all of Iwobi's limitations, we have much larger fish to fry at the moment with what funds we have available.