I agree.
If we can read anything into how BR would have acted then I'd expect them to keep putting new stock on the "flagship" routes and cascading downwards - e.g. the Deltics on the ECML replaced by the HSTs, mainly replaced by the 91s (the life expectancy at the top table being relatively short)... at the same time the MML was taking the cascaded HSTs and the Deltics were cascaded onto the Transpennine for a period.
Under the privatised railway, each franchise can order their own stock, so "secondary" routes like CrossCountry, MML and TransPennine routes have all had their own brand new stock (Voyagers, 222s, 185s etc), rather than waiting for the "flagship" trains to be cascaded away from "flagship" routes.
Similarly, the high profile Glasgow - Edinburgh route got twenty year old Class 47/4s under BR (rebranded as "47/7" and given a stripe of Scottish blue), to replace the 27s (that were themselves partly cascaded from the Great Northern) but under privatisation it's had 170s and now 385s, instead of waiting for older units to pass down the food chain.
The "problem" is that the ECML/GWML have endured with HSTs for much longer as a result - a mixed blessing as it's meant that "secondary" routes like XC/ EMT/ TPE have had much younger stock than they'd otherwise have had, but has meant that the flagship routes in the UK have struggled on with stock approaching forty years old.