I don’t think they killed him so that Carrie could “grow.” I’m not 100% sold on Carrie exhibiting a lot of growth this season–not as much as Gail and Ashley see, I think. I do think her behavior re: her bipolar disorder is different than in past seasons and the character overall is as engaging as she’s been to me in years.
I think the reason they killed Quinn is because they didn’t see a future for him in the show that they wanted to write. That killing Carrie’s “person” would take a wrecking ball to her life is a natural consequence of such a decision but I don’t believe that’s the reason they made it. There are plenty of other ways to do that without killing Quinn.