That is the sort of operation that I had imagined...For example, e-Catering on Indian railways is already a thriving concern with more delviery stations coming on line as time passes.The way it's going, you may not be so far off in your prediction...Maybe they will just make things an old SP automat car and if the vending machines break down, too bad. Hope for a burrito lady. [emoji37]
If the worst does happen, I believe that will present great opportunities for enterprising entrepreneurs to fill the vacuum with some kind of alternate service...
Such as an 'app' on your smart phone that would let you choose from an excellent menu, order ahead, and have hot or cold meals delivered to you right at the platform on some of the stops enroute. Such a plan could work, even if the train were delayed. That would work out better than having scheduled meal stops at stations, like in the Harvey House era. And it would offer much greater choices...
https://www.travelkhana.com/tkblog/tag/e-catering
It is probably a little harder to operate such a thing profitably when there is only one train or less to serve per day though.
As for Amtrak objecting to vendors serving their patrons in that manner...how could they, and why would they, as it would add to their customer's travel experience, in light of them not deeming it feasible to do itself, any longer?
Even now, I believe companies like Uber are already in this business, or starting it for home restaurant deliveries. No special catering would have to be set up exclusively to handle this...just ordering from existing restaurants and delivering it trainside would be all that would be required...
And it could be done whenever the passenger feels hungry, regardless of where the train happens to be at mealtime.