True Polymorph and legendary actions

Quote from Natrel >>

You misunderstood what I meant by "based on". I meant "proportional", not equal. Limiting the CR to "half the player's level" is still based on level.

In that case, I still disagree. People do not always use their highest level spell slot, because they might have something else they want that slot to be available for. Of course, that doesn't matter for 9th level spells, just for regular old polymorph, but the core issue is that non-cantrip spells are not supposed to scale on caster or target level, they're supposed to scale on spell level.