It is surely possible to make sure an Enviro400MMC is an appropriate vehicle for a long distance service, allowing you to convey schoolkids and shoppers over medium distances as well as commuters and tourists over longer distances, with nobody left behind or standing, despite the less frequent service.
Ordinary street stops in assorted towns and villages, with express running on strategic A roads inbetween. Anything else is surely not a "bus" service at all. It's rural for long stretches, but not remote.
One trip I made on such a vehicle was astonishingly comfortable, even though I was on it for a very long time, most definitely using it as a day tripper. I had dreaded the approach of a "city bus" for such a journey, but wow. How wrong I was.
The most obvious reason was indeed simply the seating. Not high backed, but an individual seat, with nice thick padding and plush moquette. Someone had clearly put a lot of thought into what would be most comfortable on a long journey, and was ensuring it stayed that way, day after day, year after year. It was also the first bus with a genuinely visibly clean seat I'd been on in years. I'm talking hoovering, and right into the corners.
I suspect it probably did also have a more approptiate engine and gearbox option for faster running, but if that puts off an operator, they're in the wrong business. The route is long enough that even at this less frequent frequency, you can have a small batch of such buses dedicated to it, and cover peaks or failures with more standard buses. I suspect they do have a full PVR, with interlopers only for MOTs and such.
The buses were most likely bought for the route, maybe even refurbed second hand. The route had notably seen all manner of different types over the years, including proper coaches and high spec single deck buses, then any old rubbish after Covid, until seemingly finally settling on this type.
Ironically it may have only been the previous use of costly and much more bespoke vehicles, things you couldn't hope to have used on other work, that boosted passengers enough to make sure this route survived Covid, into this pretty much exclusively Enviro era.
It is well known and well used, despite being, to any casual observer, just another bus service now. All prior attempts to brand it felt a little pretentious. You have to know your customers. Your area.
I also think it had a lot to do with the opco/depot in question being far more diligent about maintenance, as reflected by the bus being far smarter and cleaner than its city cousins working for the same group. Perhaps because these were their buses. Pride. Hence, as best I recall, no annoying rattles or squeaks. Perfect suspension.
Related to that, perhaps it had regular drivers, ones well versed in the route, the roads, and the many transitions between A roads and country lanes, the potholes and traffic issues. Where and how to drive the vehicle, so you don't draw attention to yourself as some kind of novice, moron or someone just phoning it in. The fundamentals.
Quite obviously, being well planned to allow for such running, is also a factor. It ran fast when appropriate, not all the time. The driver was in no rush, nor was he artificially wasting time (my pet peev). This was not an off peak run either, we hit the schools rush on the way back.
Maybe he drove so well because his work was valued and he had the appropriate support. It is notably one of those routes with a full ten minute layover at a bus station, technically being a mutli-registration. I am quite sure he left the bus for a comfort break. Or it was two different drivers, but they were so good, you didn't notice the change.
Given the route and other passengers, I loved the fact I could get upstairs out of the way, bagging the front seat for the views. I don't recall any lurching. It is of course an aquired taste, fast running on a top deck, but the seats and quality of the driving make a lot of difference, as I know from having been on standard deckers on a short city expresses driven by idiots.
It was a a very nice trip. Had it been a coach, I'd have not had such a great view, perhaps not even gettin a window seat at all. Had it been a Citaro, I'd have had a sweaty bum from the leather seats, and been closer than I like to be to noisy teenagers and kids in prams on a long journey.