Absolutely a microfleet- but in heritage operation (both mainline and preserved line) that would have mattered less.
It truly is a great shame that the report that was published in the Railway Magazine shortly before the Blue Pullman trains' demise, that ten vehicles had been secured by a preservation group in order to operate land cruises, proved to be false, or at the very least, came to nought. A Blue Pullman set would have been superbly suited to working luxury land cruises on the Scottsh Highland lines, and I could envisage such a venture continuing right up to the present day, in a very similar manner to the Royal Scotsman train, which has endured for decades.
From a commercial perspective though I can appreciate why BR saw fit to withdraw them when they did. They were only a small fleet, and the imminent squadron introduction of HSTs across Britain's key main lines within five years of their withdrawal would have also seen them usurped in standards of passenger comfort. The then new air conditioned mark 2s had already introduced some of the pioneering features of the Blue Pullman trains to normal passenger services across Britain and surely offered an equivalent standard of comfort, reducing the novel value of the Pullman's mod cons when many of their features could be made available on the majority of Intercity services.
If BR had been able to find further use for the Blue Pullman trains, and I occasionally entertain notions of them being put into use to form the backbone of what are now Crosscountry services between either the North East or Manchester and the South West or the South Coast, then I imagine that they would have managed another thirteen years in service at most, as during the mid eighties BR tightened up a lot of HST diagrams to allow them to be redeployed onto other long distance routes, and the Sprinter programme kicked off, so if the Blue Pullman trains weren't at that point replaced directly by displaced HSTs or new Sprinters on whatever services they would have then been working, then there would have been plenty of loco hauled stock going spare to replace the handful of trainsets. I would think though that survival into the mid eighties would have guranteed a Blue Pullman set a place in preservation, but sadly it was not to be.