Or perhaps it's a sensible availability target when factoring in appropriate maintenance regimes. All too often, the target availability for fleets is incredibly pressured and leads to a reactive regime rather than a proactive one. I don't think you can hold the LNER HST fleet up as a good example of maintenance regime - the condition of vehicles heading to EMT from LNER has generally been dreadful.
The LNER HST fleet was going off lease though, and the reuse was a last-minute DfT idea. Clearly, they were maintained in a much better condition up until the end was coming.
The fact that a whole unit is out for ages as a result of a low-speed shunting mishap does demonstrate one of the main downsides of units over HSTs / Mk4 sets - with them any damaged vehicles would most likely have been swapped out for spares and the set continued in service (although in the cae of Mk4s that may have meand short-forming due to the lack of enough spare vehicles).