There are only two years left on his contract and dude's putting up very good numbers. In other words, we see no sign that he's slowing down. He also doesn't have the extra season or so of post-season games that someone like Rip or Kobe has built up over the years.
The problem is that once a wing is past 30 you never know when their playing just dives off of a cliff. Jackson has played some minutes over his career and hasn't been known to exactly been a gym rat, either. 3 years left on that deal is really toxic, and Jax plays losing basketball half the time and being as old and seasoned as he is, I see no indication he's just suddenly get it once he's here. If you compare Jax and Miles, Jax scores more PPS and has better 3-pt and FT shooting percentages, but Miles has a better PER and a better +/- while being vastly less experienced, nearly 10 years younger, and guaranteed to only get better or not get worse for a long time while you can only guarantee Jackson will get worse. I'm not saying Miles is the answer, just a comparison to show my doubt that Jax is.