OT, but are you referring to the Wright Cadet-bodied DAF/VDL SB120s that were sold through Volvo as Volvo Merits after the latter discontinued the B6BLE?
DDA and replacement by new stock certainly factors (less so in the case of DDA nowadays for the most part) into vehicle disposals, but book value/vehicle depreciation, spare parts availibilty, reliability, running costs (fuel consumption, maintenance etc) and whether or not it's a non-standard member in the fleet as a whole also play their part in deciding to retain or sell or dispose of a vehicle.
In regards to First Glasgow Presidents, there are only 7 remaining (6 Volvo B7TLs & one Trident) out of a fleet that once numbered in the hundreds. Were it not for COVID/social distancing, they'd be long-gone as I noted in a eariler post.