One person saying Coastliner need newer buses because new equals good, old equals bad
The route operated is still the same one as has run for many years. Therefore the operating conditions haven't altered, perhaps gotten worse over time. There is a mix of fast interurban running and stop-start running out of Leeds and into York. The vehicle workings would be long and intensive, over 12-15 hours as will be the turnover of passengers.
All in all, that wears out a vehicle sooner than it would if it were used on a stop-start route. As the current buses have smaller engines and have to work harder just to keep going (traditional engineering logic of a large engine plodding around has been replaced by a smaller one working at maximum capacity) that means they'll wear out quicker and they will fail more frequently even if properly maintained.
So the company does need to replace these vehicles some time soon as they'll have been working the route for at least 7 years come the end of 2024. Logically you'd send them elsewhere in the business onto less intensive work - though returning off lease seems to be the favoured option at Blazefield leaving a nice rump of buses that simply get older as time progresses. If the company doesn't replace the buses, then presumably it accepts the route will decline in patronage and reliability terms as people make alternative choices because buses can't be relied upon to turn up when the timetable says they will?
Once upon a time, Blazefield used to run a mix of double decks and coaches on the Coastliner route. The doubles were targeted onto heavily loaded workings whilst the coaches handled the spin offs which didn't go to Scarborough. Concessionary fares have played their part in the company needing to double deck everything which ramps up the cost of operating.