I certainly understand where a lot people are coming from particularly pertaining to S4B - S8.
I'll NEVER understand the S3 - S4A complaints, however. Even back when I was on the "Phoebe Hate Train", I still think Phoebe was Phoebe in S3-S4A.
That being said and done, I actually understand Phoebe more, now and relate to her more than any of the other characters. The only time she annoys me (though, it's understandable) is her behavior towards Cole throughout S5A. Otherwise, I think she's fine throughout the rest of the show including her "baby craze phase" (which, IMO, is a symptom of her losing her own child, but I digress). We see even S2 Phoebe (who is beloved) is pretty obsessed with Baby Matthew in S2E11 "Reckless Abandon". Along with other early Phoebe interactions with kids such as Aviva in S1E7 "The Fourth Sister", Nathan in S2E12 "Awakened", Jenny in varying S2 episodes, etc. It's foolish to think Phoebe doesn't like kids, when her character (the most) is hanging around them (especially in earlier seasons, particularly S1-S2). There are S3 examples (like Kate in S3E3 "Once Upon a Time"), but that may not convince the "Anti-S3+ Phoebe" crowd, but I digress.
I do believe it was a natural progression for Phoebe to become Queen of the Underworld. There have been so many layers and hints towards a "Dark Phoebe" and A LOT of it was actually explained throughout S1-S2. Episodes like S1E15 "Is There a Woogy, in the House?", S2E2 "Morality Bites", S2E14 "Pardon My Past", etc., that it shouldn't come as a shock when Phoebe turns. With the exception of S2E14 "Pardon My Past", "Evil Phoebe" doesn't want anything harming her sisters. This gets re-established, AGAIN, in S5E12 "Centennial Charmed".
I would even argue that Phoebe became LESS selfish in the S5-S8 section of the show. What a lot of the fandom (unfortunately) fails to pick up on regarding Phoebe's "love for magic" is actually a "love for power". Which is why her character is mostly seen as "the evil one", in the early Charmed years, whenever we see an alternate timeline (or even past). And was constantly the one breaking A LOT of the "personal gain" rules, fairly early on. Once she realizes the TRUE cost of "having power" and that addictive behavior towards it is why she was so cold towards Cole, in S5A. Very understandable, though.
And why Phoebe was no longer interested in magic is because she no longer craved that desire for "power". As her S5-S8 self states MULTIPLE times, her similar S2E1 "Witch Trial" Prue realization that being a witch no longer carries the "perks" she anticipated.
So, what happened to Phoebe is she grew up. She grew out of needing to be a witch in order to feel validated, in life. Prue even calls this out in S2E17 "How to Make a Quilt Out of Americans" that Phoebe is still young, here and so Phoebe's statement in "wanting to always be a witch" is extremely short-sighted. Which is ironic coming from the one who sees the future LOL
Anyway, I think Phoebe gets a little too much hate, and I think Phoebe is a wonderful example of a young adult growing up.