All but Jo came from wealthy families and they wouldn't have needed or wanted roommates past a certain age. And they all except Tootie, wanted to move out at certain points. Blair was going to move into Gamma Gamma, Natalie got an apartment that Tootie crashed in, and Jo got an apartment, but then they made the attic into a room for Tootie and Natalie.

All but Jo came from wealthy families and they wouldn't have needed or wanted roommates past a certain age. And they all except Tootie, wanted to move out at certain points. Blair was going to move into Gamma Gamma, Natalie got an apartment that Tootie crashed in, and Jo got an apartment, but then they made the attic into a room for Tootie and Natalie.

Mindy Cohn asked the same question. She said when "Over Our Heads" started, the show jumped the shark.

Agreed it was especially ridiculous to have Blair living with the other girls the last few years. She obviously came from money and was older than Tootie/Nat. Considering Jo's family was blue collar it would make sense she would live with other girls.

Sometimes you just need to suspend disbelief when watching tv shows. Their living arrangement didn't make alot of sense but the roommate dynamic was pretty crucial for the backdrop of the show.

It’s ironic that it was obvious in”The Little Chill” that (even though they were still good friends), Nancy, Sue Ann, and Cindy all went their separate ways with careers, etc. - yet Blair, Natalie, and Tootie remained stagnant.

even though Natalie and Tootie came from wealthy families that doesn't mean that their parents would pay their living expenses as adults beyond what they would normally pay for tuition, room and board at college and maybe a little extra and yes Blair could have afforded to live on her own but her bond with Mrs. G and the other three girls was really strong and she would rather live with them and enjoy their company than live on her own.

In the last two seasons, the show countered the not so realistic living together arrangement by having all 4 girls being busy with work, school, and life. There's not that many episodes where all 4 girls are interacting and together. Very noticeable in the last season.

It’s the ultimate "sitcom logic" puzzle. You hit on the exact moment the suspension of disbelief started to strain: "The Little Chill." Seeing the "lost" girls move on to realistic adult lives made the core group’s refusal to leave Peekskill look like a case of arrested development.

The transition to Over Our Heads in Season 7 was the "shark-jump" because it forced professional women into a communal attic, but the show used a few key (if flimsy) anchors to keep them there:

The "Partnership" Trap: The writers pivoted from a school setting to a business collective. By making the girls co-owners or essential employees of the boutique, their living arrangement became a "work-study" extension. Blair, despite her wealth, stayed to protect her "investment" and practice her business degree.

Tootie as the Anchor: Because Tootie was younger, the show used her remaining time at Eastland/Langley as the "gravity" that kept the older girls nearby. Since they were a "found family," the writers bankered on the idea that they wouldn't abandon the youngest member.

The Attic as a "Penthouse": By renovating the attic into a trendy, multi-level living space, the producers tried to make the communal living look like a choice of style rather than a lack of funds. It was an attempt to mimic the "cool" roommates vibe of shows like St. Elmo’s Fire.

Mindy Cohn's Perspective: Mindy has been vocal that the show lost its "Facts" (the actual lessons of growing up) once they became shopkeepers. In her view, once they weren't students, the show became a standard sitcom rather than a coming-of-age story.

Ultimately, the "roommate dynamic" was the brand. Had Blair moved to a penthouse and Jo to a Bronx walk-up, the show would have had to become an ensemble drama like St. Elsewhere to follow them all.
Since you mentioned how noticeable it was that the girls weren't together as much in the final seasons, did you feel like the Pippa and Andy additions were just "filler" to make the house feel full while the main stars were off filming their separate storylines?