I'd imagine that they would eventually in the next decade be replaced by a universal Sprinter replacement for all the rural/scenic lines to simplify the fleet.
I'm not totally sold on the idea of "scenic" trains (given the costs associated with micro-fleets, the need to make railway lines function all year round rather than just being the playthings of tourists during school holidays - fine for preserved lines where people will pay for such things but a bit of a luxury on regular services).
However, I think that there's merit in considering the total DMU fleet at ScotRail once the current franchise finishes in seven years time.
Maybe the HSTs will be replaced by something like an 802 in the next franchise, but that still leaves maybe two hundred DMU carriages for routes that aren't electrified in 2025.
A lot of them will be fairly functional, given the stop/start nature of commuter routes around Edinburgh/ Glasgow, so the bulk of the DMU fleet will presumably be tailored for routes like the Fife Circle and whatever "Southside" routes remain unelectrified.
Maybe one fleet for such routes and another for the "scenic" lines (tailored for longer distance, less urgency to have "commuter" doors etc), but where do you draw the distinction? Is the Borders line "scenic" enough? Or does it function as an extension of the Fife Circle for operational purposes? Less scenery in Cowdenbeath, maybe, but I'd argue that the line from Dalmeney to Kinghorn is one of the finest half hours you can spend on a train (as long as you are looking over the Firth of Forth).