Notwithstanding bits of Mk3 and old HST still hanging around, I suspect the 158s and 165/6s will be around for a good while yet. Northern, W&B, Scotrail northern routes and Great Western look the most likely to be around.
I imagine EMT will, as suggested above, do a wholescale replacement of their fleet, seeing cascades of 170s or similar look unlikely.
I think we all know Northern will end up with whatever no-one else wants, as we can see by the pacers being replaced largely with 158s and 170s from Scotrail.
The south western branch lines look likely to retain their 150s for a good while yet too, not much need to replace them for a good time, they seem to be doing ok.
It also wouldn’t suprise me if an open access operator got hold of some old HSTs for tuppence and refurbished them, so it may in the end be HSTs that last the longest (although I wouldn’t be suprised if the Mk3s are replaced 1st).