"The only downside of Act II is that Elphaba runs away with Fiyero instead of Glinda."
One thing that Gelphies don't seem to realise is that Glinda WOULDN'T run away with Elphaba. She could ask Elphaba to give up her plans and stay with her (and Elphaba would, if that didn't mean giving up her morals and becoming passive to oppression), but Glinda would not give up her life style for Elphaba, or anyone for that matter.
Like, did you guys not watch it? Glinda had the opportunity to run away with Elphaba several times, but she never did. Elphaba asked her to come with her during Defying Gravity, Fiyero asked her to run away with him so that they could look for Elphaba together and be with her during Thank Goodness, and the way they're talking to each other implies that that wasn't the first time they had this conversation and it also wasn't the first time she chose a lie over the people she loved.
Also, Elphaba and Fiyero were more useful to the Animal cause dead than alive, that's the conclusion they come to at the end, that's why Elphaba makes Glinda promise not to clear her name, because Glinda was the only one who had the power to do something, the biggest power of all: popularity (I've already talked about how every single word Glinda sings in Popular is true). People love Glinda, but they hate Elphaba too much to keep loving her if she associated with her, like they loved Fiyero and now hate him because he chose to love Elphaba. But people could love the Animals if Glinda told them to, so riddle me this: if Glinda left with Elphaba, who would save the Animals? Who would do good? Who would change things?
Elphaba and Fiyero left, not only to be together and in love in peace, but also because they were doing more harm than good by staying. Glinda is the rising Sun of a new Oz. Staying alone and having time to be just by herself and reflect on her actions and finally being Glinda, the Good for real by being a benevolent leader, who stops the Animal genocide and brings Oz back together is her redemption arc. It might not be the happiest ending, but it is the best possible ending for her!
