That is what London Transport reckoned for their buses, and some of them did a lot more than that. The demise of these buses was the layout, Not the mileage. With the cab entirely isolated from the passengers, this made a conductor essential, and to save staff costs the replacements had the entrance doors beside the driver who drove and took fares. Nowadays the emphasis is on "modern" buses so longevity is no longer particularly important.
A lot depends on the driven life of an engine. I bought an ex-minicab, a petrol driven Rover 3-litre which had spent its whole life shift working; one driver doing days and another using the same car for nights. The company aimed to get the ferry and airport runs for preference, because there was usually a return fare. So the engine never got cold, and was rarely switched off.
When I bought it, it had a quarter of a million miles on the clock, and I put another 120,000 commuter miles on it before I sold it (Mortgage interest rates and petrol prices shot up at the same time, and I ran out of money; otherwise I would have kept it).
I didn't do anything to the engine apart from services, and as far as I know the cab company didn't either. A third of a million miles on a petrol engine isn't bad going. I saw it around for over a year after I sold it so the new owner obviously didn't have too many problems either.
Jim