I know that /co/ like to meme about Star committing genocide against magical creatures, because some people need to find something in every finale to bitch about, but to me it’s clear the show itself presented a good case why magic must be destroyed, even if it was not explicitly stated in the finale:

First, magic was once again corrupted, which would entail magic being on the fritz again, and just like in season 2 that means spells becoming deformed, useless and outright dangerous. The only way that damage was undone was by Star sacrificing herself under very particular circumstances. It’s unlikely she, or anyone else, could pull that off again. If anything, destroying magic was merely sparing the spells from the horrendous fate that awaited them.

Second, the Spell with No Name was established to be able to destroy the entire multiverse if left unchecked. During her battle with the Solarian warriors Eclipsa used that spell, which was shown to fly away destroying everything on its path. It’s strongly implied that spell will be active indefinitely. In that case, pretty much the only plausible way to prevent reality from being destroyed was indeed getting rid of magic.

Third, Mewmans have been shown to be reckless in their use of magic, developing potentially world-ending spells like stopping gravity and time. On top of that, the MHC was shown to be power hungry and untrustworthy. Even if Star was able to fix the current situation some other way, who knows what kind of havoc someone with the wand could unleash in the future.