You're assuming that a culture as ossified as modern wizarding society has lots of people striving to create new spells en masse, and that it's sufficiently easy to do so that they'd be getting created if the people were trying and "slots" were available. That's absurd. Also, you seem to have no understanding whatsoever of the Interdict of Merlin - it says that spells above a certain power level can only be passed between intelligent minds, with no intermediaries(like books) allowed. The best I can assume is that you're engaged in circular reasoning, with the lack of new spells proving the slot theory proving your weird interpretation of the Interdict proving the lack of new spells.