What's with your weird obsession with edgy and disgusting endings and label them as "dignity"? Sounds like school shooter logic.
Miss Heinous died at her own mother's hands after conquering Mewni (that's already a more tragic ending than Toffee's), but all the magic accumulated in her body proceeded to save enough energy to maintain an infant form with reseted memory.
Heinous is gone, this is a completely new character.
So now we can see a "what if" of Meteora's character from stage 1.
The idea is interesting and shares the show's idea of grey characters, whose evil ways are based of circunstances which can be altered. Far more dignifying than the uncomfortable guro porn with babies your sick mind wants so much.