Reasons why moden SMG4 has become mid:
1. WOTFI is no longer about SMG4 vs Mario. Now its part of the "Story arcs".
2. SMG4 was about dark humor and absurdity. The moment SMG4 became "serious" and anime-esque all of that changed.
3. SMG4 originally was intended for mature audiences, and its original adult and absurd humor helped the series stay that way. When SMG4 got "serious", the series became less adult oriented and now its attracting a lot of kids, which shoudnt even be watching that stuff.
4. The rainbow colored marios/Mario 64 recolors where charming and memorable in the series. Now the rarely appear. SMG4 and SMG3, who whwre supposed to be Mario recolors, now have become different.
5. Most of the story arcs are very mid. The only exception being the Waluigi story arc.
6. Melony was just added to attract weebs and simps.
7. The new gang is very awful, Boobkins being objectively the worst character.
8. Meggy used to be likeable, now she is annoying. And she was better as an inkling.
9. SMG4 rarely uses Super Mario 64 elements or Gmod ragdoll physics in his current videos. Which where part of the charm of the original series.
There are more, but this are some of the main ones.