I think that's a rather harsh assessment lederhosen.
Certainly so far as Leeds was concerned NS and later BA Regional carried on in seventies in much same vein as BKS in it's later days. The latter might have been more fleet of foot in fifties/sixties but by time Viscounts were on scene it was pretty set in its ways.
Withdrawal from Leeds came when Viscounts reached end of their useful without a suitable replacement. While the runway was a factor at Leeds BA also pulled out of (IIRC) Cambrian's Bristol and Cardiff territory at same time.
British Midland stepped in at that point on LHR (only) but they never competed with BA on the route and independents took over all the other ex BA LBA services.
BMA competition on LHR trunk routes to GLA/EDI came later as part of Thatcherite 'free market' reforms.