I think your spot on about because they were base models they were more likely to be bought buy fleet buyers and people looking for a cheap car, hence the poor survival rate. Mind you, if that were true then I expect we would be seeing more TCs on the road than SDLs as they were more likely to have been bought by people who are interested in cars. Or, just maybe, it may be a combination of the two, with DLs being thrashed as cheap cars, TCs being thrashed by more 'enthusiastic' owners and SDLs sitting somewhere in the middle, or of course it may just be that the SDL sold more than other models.
One general trend must be true though: As Marinas became very cheap transport it would have been the TC and higher spec (SDL) models attracting buyers rather than the base models, as a result a lot were scrapped. This appears to be the same case for any classic car and often even by the time the car reaches classic status the base models still aren't recognised as being worth preserving by some owners' clubs, for example Dolly Sprint owners turning their noses up at 'lesser' models and let's not get started on the classic Ford world, wonder how many base model 2 door MK1 and MK2 Escorts are left now?
So glad the MMOC isn't like that