It's hard to say how long SG development time will take.
I have not seen anything from Ocean saying how long WIAB will be, and he is developing both concurrently.
If WIAB is materially shorter in chapter length compared to SG, I don't know if he would change the rate of SG chapter production to WIAB chapter production from 1:1 to perhaps 2:1 for example.
His hardware has also limited his vision for scenes so how much will the hardware improvements he just identified in his dev log speed chapter development?
I do know that I'm sticking around for the duration - the story/render/music combination is First Class and, IMO, worth waiting for.
As far as additional rework, he's mentioned nothing about this beyond the work that he is wrapping up for Season 1.
He has said that he is shifting to blender (starting with season 2, I believe) and this will offer him more fidelity when working on his models.
I'm not a tech expert so I can offer no qualified opinion about what a new generation of Genesis models would bring to the table.
My impression from the little reading I've done is that it would be evolutionary versus revolutionary so perhaps Blender will more than compensate for the evolutionary benefits of a new gen of Genesis models.
I trust Ocean will continue to keep us informed regardless!
Cheers!