That’s actually such a good idea! :D
Although I’m pretty sure that Cozy Glow was already evil to begin with. But she was actually—perhaps even without her knowing—about to grasp the real meaning of friendship through the CMC.
And yes, to be fair, if Cozy Glow was actually imprisoned in stone all the way up to Sunny’s life, then that would actually be even crueler than to have her been freed only a few years later. I’ve read a fanfiction with Twilight Sparkle having been the one to free her herself, and she herself taking on the responsibility of caring for Cozy Glow, and even admitting that she was wrong being penitent, vulnerable, and humble in front of Cozy Glow.
That of course did wonders for them XD