Sword of Spirit
Legend
The reason I say it makes no sense is that it is unnecessary to achieving the goal and adds complexity while introducing what I see as a problem. He had already achieved the goal by adding a scaling save bonus and simply keeping the class level scaling DC. Changing that to a spell level scaling DC has the effect of seriously limiting the usefulness of low level spell slots--something that isn't mentioned in his goals at all.I don't see how it makes no sense. It's just another way of making sure that low level spells aren't as effective as high level ones, an assumption that D&D has incorporated in various ways for a very long time, particularly strongly since 1989 with 2e.
I personally prefer that low level spell slots stay relevent. As has been mentioned, when you cripple the effectiveness of low level spell slots you tempt a 5 MWD mentality.
So why do what is suggested for the reasons suggested? It makes no sense.