Great thread!! With Maria coming back in 1974-7 was all about money; she was bitter about missing the financial boat of pro tennis - and who could blame her? I remember seeing two of her matches at Wimbledon in 1977 when she was 37 (which actually doesn't seem anything like as old now as it did at the time!); she beat Janet Newberry, then the Italian Open chamion 1-6 8-6 8-6 before losing to Billie Jean King on Centre Court, and I still have newspaper clippings about that match, when it was generally acknowledge that she was a shadow of her former self, just showing glimpses of her old form. I think I read somewhere that even with her moderate success on the tours those years she made as much money as in her glory years - so in that respect her comeback was reasonably successful. Even today I rarely read anything about Maria Bueno now without there being references to her lack of financial success, and of course that's applicable to all pre-Open tennis stars but the timing of Maria's heyday makes it obviously more painful for her!
Many year later (in 1989) I recognised Maria Bueno in a post office in Battersea in South London (really!) and I approached her and we had a talk. I told her I'd obviously missed her Wimbledon-winning years but that I'd seen two of her Wimbledon matches in 1977 and I trotted out the score of the win over Newberry and I got the impression that she was pleased that her performance in the later years had stuck in my mind, which it had!!
With Tracy Austin I think the 1993-4 comeback was all about missing the glory, rather than the money. In her early-90s autobiography she boasted that she thought if she came back she could easily reach the Top 15, but the reality proved otherwise. She pulled off a couple of good wins (I remember she beat Gigi Fernandez and a Maleeva - Katerina?) but an Eastbourne humiliation to Kristine Radford was enough to shame her into hanging up her rackets for good!!!