It definitely does make a difference! Arriva's fleet overall has fallen well below the standards of First, Go-ahead, Stagecoach and many mid-sized groups and operators.
Look at the investment Transdev are making in their more important routes (and some of their less important routes as well!). First and Stagecoach have made huge investment in moves to electric.
I'm not saying that 16-year-old Geminis don't have their place, they're absolutely fine for shorter or lower profile routes, but it's hard to imagine any of the other fleets would still be running them on a key route like the 415, at least not without imminent plans to replace them with new buses.