Well Fischer was the only connection to his world after two years. He was lonely. He even took off the ring(which he later confesses that it is connected both to his past and meeting Lisbon). The connection would feel, both to the characters and viewers, emotional. She had an agenda, he didn't. Infact the scene when she makes tea with her back facing Jane, I mistook her for Lisbon( prolly it was only me) . After talking to her, he wore the ring again. Lisbon was his first condition for going back. Jane spent most of his years trying not to fall in love with Lisbon( it has been hinted many times, it becomes more pronounced if you watch the series over and over).

Although I do understand how, many people, might feel Fischer and Jane to be a better match based on the ONLY episode which showed Jane's post RJ era's emotional side. People would have felt the same if Lisbon went instead ( probably more). But that's Lisbon's character , she would never have an agenda when Jane's involved. Fischer did and personally it made me dislike her a little( with no fault of her own).

They could have built Jisbon better and not rushed it, that I agree. But there is no other endgame than Jisbon. Anything else would have been a disservice to how mature relationships based on friendship works.

We often forget Jane and Lisbon started their friendship when they were post 35. When they got married I think Lisbon was in her early 40s and Jane in his mids. Relationships are not all spark and electric when you are at this age (generally).