Firstly my info about Transdevs fleet comes from Buslists.uk and UK buses. I didn't say that the 100 double deckers need all be replaced by NEW vehicles, but bear in mind say 8 y.o used deckers if mileage required is low. However these would need replacing somewhere along the line. But leasing and returning after 7 years leads to polarisation. Arriva could face a similar difficulty replacing the large number of buses from circa 2010 it runs, although leasing hadn't been an issue.
So who pays? The customer, with higher fares to offset the higher operating costs, the council, with a higher tender price or the drivers by having a pay cut (good luck with that one... not met drivers who willingly accept pay cuts to replace a fleet). What you won't have is these 'new' buses for nothing, there's a cost to doing so. A newer vehicle is also more mechanically complex than an older one, so when it goes wrong it requires specialist attention.
Arriva only replace buses when their operating companies achieve the required profit margin. As it's not been achieved, DB has refused to invest in new vehicles. It has however managed to pay dividends back to Berlin. FirstGroup also expect a new bus to generate passenger increases when they deploy them. No passenger increases, no new buses for you.
If you have lower mileage work which justifies a well maintained double decker, then you can run it until it finally expires - or becomes uneconomic to repair as you can no longer obtain spare parts. My local bus operator, Southern National ended up with a fleet of Bristol VRTs, in excess of 20 years of age working school/college movements. So much so it became one of the enthusiast go to places to see them in squadron service. Nothing wrong with a set of decent reliable buses on lower mileage work if that retains the services.
I'd take ukbuses info with a massive pinch of salt. It's user edited, like bustimes.