Because Jaya is about 3000 and doesn't have things like demons to extend her life.
In this hypothetical new!Mending scenario, we could write it so that age catches up with the Oldwalkers slightly slower. This ultimately doesn't change too much down the line (Alara onwards, it wouldn't change anything I think) and could make for a decent "passing the torch" story. You could even keep Chandra, have her actually meet Jaya and focus a little more on the part of her story where she just isn't as good as Jaya.
It seems like the lorwyn block takes place a good amount of time after the mending. Lilliana has her tattoos so we know she already made her pacts, and I remember her origins article stating that she was heckie old by the time she made the pacts. It would make sense if Jaya was dead by know.
That doesn't explain Venser tho. Or Elspeth
I wouldn't actually set Lorwyn on that timeline. Hell, I'd probably wouldn't have used Lorwyn for this. (If we still want Lorwyn, it could be set after the first post-Mending block and just bump up everything by a year.) I'd set the story just a couple of years after the Mending. Ten at most. Long enough for characters to come to terms with the new status quo, but not so long that the you couldn't feasibly introduce a depowered Oldwalker in the future.
I'd actually introduce Liliana when she's getting her last demonic pact in order.
Venser had access to slow time water.
Elspeth's timeline is a certified mess though. I'm kind of just resigned at this point to the idea that the timeline is never going to make sense post-Mending
Actually, I wonder if Lorwyn could've still been the plane to introduce the post-Mending 'walkers. I mean... that seems like a LOT of balls to juggle but it's not like the Lorwyn story was anything worth writing home about anyway, so...
There's even some interesting stuff going on there with transformations from one state to another that play well with Garruk's story after his interaction with Liliana (make that towards the end of the Lormor books), and with Liliana herself, and Old!Jaya could possibly be an interesting pov for that as well. Actually the more I think about it the more I like it honestly. Hinge the story around these neowalkers interacting with some of the races of the plane and you have a narrative about players building tribal decks, essentially.
I mean I don't like... really think this would work? But it's a step closer to working than what we actually got, at least.
I wouldn't use Lorwyn, personally. The plane has
a lot going on, even if the story isn't all that interesting. Using the plane to show planeswalkers coping with the Mending would make it even busier.
Have you seen the
What if Episode III Were Awesome? thing on youtube? I kinda want to write that for the post-Mending era.