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.

Its purpose might simply have been to slow down the rate of high-level spell development and use. We know that the greatest body of magical lore would have been lost with Atlantis (going with the theory that the Atlanteans created the Source of Magic) and that magical research appears to largely be the province of individuals. The Interdict of Merlin would be like going to medieval Europe and saying "OK, you can carry on developing science/natural philosophy/etc., but you can't read any classical works, and if you want the latest research, you've got to get it in person rather than reading your fellow thinkers' texts".

Is anyone here familiar with the "Labyrinths of Echo" sequence by Max Frei (the first book of which has recently been butchered into English)? That has the central premise that magical power is fuelled by the magical axis running through the the world, and that overuse of said power has nearly drained away the soul of the planet. As such, the protagonists are a magical police force dedicated to protecting the ban on high-level magic so that the heart of the world can recover and the all-too-near apocalypse can be averted. I wonder if the Interdict of Merlin is based on a similar idea.