The 700s didn't just replace 319s
Even on the "legacy" Thameslink route, a lot of the services were operated by 377s and 387s.
That was temporary and was always stated to be temporary.
With regard to the 700 interiors, though, if there is a marked decline in commuting it might be worth the mid-life refurb adding wider seats with armrests throughout, possibly as a single class train. As I've said above, the Northern 195s, now I've used one that was properly full and standing, are just as easy for circulation if not better, because when you walk down the aisle you don't keep bashing people as the armrest "protects" them. The things that make the real difference to circulation are the door standbacks, wide doors and unimpeded inter-vehicle gangways. The standing capacity is less, to be fair, but that may not be needed in the "brave new world".
Indeed, if the Government is going to push forward with removing the F****a S*****s from the 80x as they seem to be suggesting they might, that's a ready supply of reasonable-ish 2+2 armrest-fitted seats to put in the 700s - being a regional seat, they are ideal for a regional train such as that. But that might of course not happen and there are many options with varying merits perhaps more befitting of a speculative thread.