Indeed, or you install something like a Costa Express machine (these are surprisingly good, I find the coffee from them nicer than from their stores) like some European countries* have done.
Thing is, if you're making a short journey you want to board and get a seat, and keep it. Having to go to the buffet first is a faff. Buffet (and for that matter bistro/restaurant) cars work better on very long distance runs. It's even questionable if there is any point in them on London to Manchester now (or e.g. London to Bristol), and on HS2 there'll be very little point at all for any service not going to Scotland.
The main services where onboard catering is likely to be viable now are London-Scotland including CS, the long XC runs and Paddington to Penzance. Those could even sustain restaurant facilities if done well (e.g. DB style Bistro food either for sit-in or at your seat - basically fancy microwave ready meals with e.g. fresh salad/bread added). For almost everything else, most people will buy before boarding.
The one big exception to all that is First Class breakfast on the way to a day's intensive work.
* Surprisingly, given that coffee is a big thing in most European countries, they will accept some utter garbage when travelling, such as in the Netherlands pretty much all coffee will be made with UHT "Koffiemelk" which is just vile. The UK it seems will no longer tolerate that unless it's free (as is expected in hotels). The benefit of using UHT is that it requires less care and cleaning of the machine, the downside is that it tastes so bad it might as well be off.
