I'm afraid this is historically inaccurate (and given your background you really should know better). GMT closed plenty of depots before Altrincham and Rochdale. For the record: Weaste (January 1986), Leigh (February 1986), Hindley (March 1986), Birchfields Road, Frederick Road, Northenden (all October 1986 at Deregulation). Subsequent closures were technically nothing to do with GMT as buses were run by GMBuses separately from GMPTE's area of responsibility. To continue: Altrincham, Rochdale, Swinton, Tameside (all November 1991) though Tameside's sub-depot at Glossop survived until 2009. Subsequent closures were under private ownership. I would suggest that the closures were much more about reducing the overheads arising from the very existence of depots rather than necessarily reflecting the state of the economy in the immediate environs of individual depots. The complete absence today of any large operation based in Salford is noticeable. It's also ironic that Stagecoach Manchester has a large operation in Sharston barely half a mile removed from the former Northenden depot! It did however effectively replace Princess Road depot. If there has been any trend at all, and it's hardly an obvious one, it's that the depots with the smallest PVRs are more vulnerable demonstrating the value, at least to the bean counters, of economies of scale.
Apologies, my timeline is haywire. Nevertheless, the reasons for closures were varied, with many of he 1986 closures indeed due to overheads and close proximity of neighbouring depots. Also, before dereg, city area depots were much less profitable than district depots, due to a high proportion of peak only services, higher levels of congestion and presumably higher land values. De-reg changed, with peaks levelled out (M-F shopping hours were virtually the only period of the week that didn't see service reductions overnight on 26/10/86). During GMB days - and periodically before that - I had legitimate access to the performance of both depots and individual services. From that, I can assure you that of he depots left after de-reg, Rochdale and Altrincham were constantly in the bottom two. The reasons for Swinton and Tameside's closures were very different, as both were good performers - usually about 5th & 4th respectively. Swinton depot was actually in line for modernisation, but when all the investigations were done, it was found that there had been a long running drainage problem with fuel oil seeping into the earth below. Obviously, steps were taken to immediately address any environmental dangers, but it proved financial impossible to permanently address it and so the HSE said it was no longer fit to be used as a vehicle depot. Its now, at least partly, a small housing estate. Tameside's closure was a little more mysterious. Despite its modern day problems, Tameside was historically "good bus country" albeit some services better than others. However, it is widely believed that it was closed because of a gigantic "personality clash" between the Director General and the local TGWU leadership (this was before Unite). Economies of scale are obviously always going to be relevant, but it indicative that Tameside's work had to be split between Oldham, Hyde Road, Stockport and Glossop. This lead to a lot of wasteful dead running. I was also talking to a recently retired driver latterly at the current Ashton depot (but previously, Glossop, Whitelands Road and "Joint Board"*) before that and he was saying that before Glossop depot closed, the then MD, Mark Threapleton came up to tell them straight that the reason it was closing was not its small size, but simply because of constant "24/7" congestion between the M67 junction near Hattersley and Tintwisle. He said that if they could find an affordable piece of land and get permission to open a pcv OC, he would move Glossop ops, buses, staff the lot immediately. However, they couldn't so it had to close.
