They would struggle to compete with Avanti and the airlines when the journey time is inevitably so much longer that way.If the E&G 15 minute interval service isn't coming back, does make me wonder whether they could get into Queen Street...
Though suddenly their 5 car trains won't cut it any more. If they can easily fill them from Edinburgh, carrying on to Glasgow doesn't make commercial sense.
Plus they are benefitting from having had their track access agreement approved pre-Covid. Grand Union Trains presented an economically stronger case but were rejected, having missed the boat (or should that be the gravy train... ).
I doubt any further expansion of their service will be countenanced until/unless (and it's a big if) railway finances improve.