I think most of us wish Mrs. Garrett had remained with the show as a regular during its run, however long that would have made the series, be it 6 or 9 seasons or another number.

Charlotte claims the girls "didn't need" Mrs. Garrett anymore, because they were growing up and relying on each other. Well, the Golden Girls women were certainly grown up, but in some episodes where it looks like Sophia might leave they lament about how much they need her, and are thrilled when Sophia stays with them after all. These are women in their 50s and 60s (sorry Blanche, lol) and they still glean wisdom from someone who is for them an elderly matriarchal figure (and in Dorothy's case that's quite literal).

Our Peekskill girls still obviously have a lot to learn in life, so it's a shame the writers didn't realize they could still keep Mrs. Garrett with them as an important part of their lives whom they need around them.

And as much as I enjoy Cloris Leachman, it is pretty ridiculous to bring in another middle-aged woman to live with the girls to "look after the store and the girls"---Mrs. Garrett, that's what you're for! lol If they truly felt the girls were growing and didn't need the mother figure anymore (which is ridiculous if you consider my GG comparison), then why wouldn't they have the girls really make it on their own, banding together in 4-part partnership over the store and living in the house on their own?

Oh, if only for a time machine, lol...

Charlotte recently stated in a magazine interview, that she left more due to health concerns, which is why in seasons 6 and 7 she didn't appear in several episodes.

Well, she says both (I assume you're talking about the "Entertainment Weekly" article):

Rae: Towards the end, I felt that I really wasn’t needed that much. The girls seemed to be self-sufficient, and I felt like my part had become… Well, it’s like my friend Jean Stapleton said about her character [Edith Bunker on All in the Family]: She felt like she’d done all of the different areas of the woman’s life, and she really felt like it was about over, that there was nothing more. For Mrs. Garrett, they always kept it in the same mold, but the girls were growing up, and I felt very comfortable about them being strong and powerful. So I left. But I felt good about it, because I knew that the girls were well-loved and the popularity would continue because of them and their connection with the audience.

Whelchel: When Cloris Leachman stepped in it was a totally different energy, which was wonderful, but because of that, there was never any kind of comparison. She sent notes to us even before she came on the set, so from day one it was, like, “Oh, great, it’s another chapter—and this one will be fun, too.” But when Charlotte left, I don’t think we quite understood why. I don’t know if we knew it was for health reasons or what. I mean, I’m sure she made it clear at the time, but in hindsight, I look back and it’s kind of foggy.

Rae: The producers were shocked when I told them I was leaving. They offered me millions to stay. But I thought it over and said, “What are they gonna do, sprinkle money on my grave?” Because not only was I older than the girls, but I had a pacemaker with a mitral valve. When we came back from France—remember The Facts of Life Goes to Paris?—I had it put in. Over that Labor Day weekend, I had it done, and then I went back to work. We kept it a secret, and all was well, but I just didn’t want to overtax myself. I wanted to have a career and continue to act, because it’s such a privilege and I love it, but I didn’t want to kill myself. So I thought it was time to move on. And they did well without me, didn’t they? They went on for another couple of years.

I always felt that Mrs. Garrett would have returned if there was another season because in 1986 when she left, her character would be in the Peace Corps for two years, meaning that she would be back after the 1987-1988 season. I would like to think that she did come back but the reunion movie in 2001 shot down that notion.