I can think of a few examples where parks have kept/built up more trains than they need; up until 2020-ish, BPB would always have 3 trains in the station for Big One and 3 for Avalanche, even though those rides have not operated with 3 trains for well over 15 years in both cases. I guess this is a slightly different case in that the rides were both designed for 3, but the park were still making more trains serviceable than they needed. (Sadly this has stopped now - and as a result both run with one train much more often).
Also, the Roller Coaster at GYPB has 3 serviceable trains even though it very rarely runs more than one at a time.
Perhaps it comes down to whether a longer coaster with the ability to run 3 trains would have been ideal - then, if one was out of action, there would be 2 available by design. I take onboard that buying a 3rd train without being able to run it would be expensive, but I do hope they are able to find a fix of some kind as the situation now is far from ideal.
I wasn't suggesting Thorpe had increased the Fastrack capacity; but more that it's likely that more people bought one to avoid the four hour queue, and going by the 60 minute Fastrack queue time, it did not seem that Thorpe had taken any measures to reduce the capacity to factor in the 1 train. Therefore, they could have potentially sold more than usual despite not literally increasing the number of available Fastracks.