If it was the last train of the night, how long would the wait have been for the school children? I've seen gaps of over an hour between services from Guildford to Woking and visa versa during disruption. This can occur if services to or from Portsmouth are all diverted via Basingstoke.
Very occasionally I've seen Woking stoppers head to Guildford but that's rare. I suspect it's dependant on avilanle staff, which goes back to my point about being able to vary whose disrupted.
It was the middle of the day (the children were going back to school for lunch).
Much as varying what is disrupted could at first glance appear to be "fair" there are certain services that would have very limited impact, for instance of you turn back the services from Weymouth at Basingstoke and move those passengers onto the "stopping service" that has also come up from the south coast the calling pattern would be Fleet, Farnborough, Clapham Junction and London rather than just Clapham Junction and London. Given the train is only about 7 minutes later at Basingstoke the arrival time into Waterloo would be about 15 minutes later, which is hardly going to impact many people by very much.
Therefore, always cancelling that service (or alternating it with the WofE services) could be fairer than trying to limit the impact so that everyone is disrupted over the course of several disruptions, as for most of you cancel a service they are going to be delayed by at least half an hour, although more often than not it effectively stops travel between certain Stations for a period of time.
Bally it is fairer to cancel express services target than stopping services as everyone can get where they need to, it will just take slightly longer.
One trick that appears to happen, at least on stopping services, is that they are shown as running, then running late until almost the last minute before being cancelled so that passengers don't ask for a taxi. I know if another case where someone was stuck at Basingstoke during disruption and had to get a friend to pick their child up from childcare because of that (both this and the school children incidents were within a year of each other, and given how rare major incidents are on the SWML that is above average of disruption is supposed to be equally spread over services).