I guess those windows will become a thing of the past when the railway becomes penalty fare zone over 100% of the routes - which I suspect is the general desire of the DfT and parts of the industry, with the unfortunate side affect of treating everyone as a fare evader when not everyone is - and the general outcome being that for those who are not fare dodgers it's such an unpleasant experience customer service wise that you retreat to the comfort of your car and revenue overall goes down...
There are still hundreds of minor stations where you cannot buy a ticket, (or where a machine exists, but does not sell full range of tickets) which is why there are excess fare windows.
Where it gets a bit customer unfriendly is where the timetable offers a cross platform change, but the excess fare window has been moved to just inside a gateline. And then you get some stroppy guard who says you shouldn’t have changed trains without the detour to window even if it means waiting an extra hour to next train.
But I agree, if treat passengers badly or like a criminal, not likely to be keen to travel by train again.
Regarding the Op, if he had got off train to buy another ticket, would it have been practical to do so, without long delay. That is problem with no guards and no customer friendly attitude.