You never know, and it's still a good possibility that she's alive. Making her playable would be just the kind of thing that absolutely nobody would see coming but which a lot of people would be happy about, so SE can't really lose by doing it. And why would they lay the groundwork for her to have the reasons and connections to make a journey away from Midgar if they weren't going to follow through with it?
Also, by the end of FFVII's traditional story route, there are only two women left in the group. Tifa has only one female friend left, Yuffie, but the age difference and her brash personality might not make it easy for her and Tifa to ever be very close. Letting Jessie join the group ultimately adds another girl to the endgame cast, someone Tifa can relate to more easily because she's already been friends with her for a long time, and allows us to still be able to have a choice for Cloud's love interest after Aerith's gone so that it doesn't feel like the only reason Tifa may have ended up with him was because she just happened to be the last woman standing.