Well, she just COULDN'T end the book with Meg saying "I asked for all kinds of splendid things in my castle and all I got was this lousy husband." Readers would have revolted! (And it's not really a fair description of John, even if he doesn't make most hearts pitter-patter.)

I suspect that Louisa is gritting her teeth and Giving the Readers What They Want, which is all the characters paired off like salt and pepper shakers and having adorable babies! She sneaks in the bit about not relinquishing all artistic hopes or confining oneself to helping others fulfill their dreams of beauty as a salve to her own conscience, which wanted to make Jo a spinster writer.