But if I book a work trip LHR-JFK-LHR and then decide later to go on holiday immediately after that LHR-HEL-LHR then I'm not taking advantage of cheap ex-EU fares, I can't combine the trip into one PNR because I need to keep work/personal separate, and BA is acting like a low cost carrier by making me pick up my bag in LHR and rechecking it.
I agree that the non-acceptance of separate PNRs is frustrating. But the distinction remains that unlike the typical LCC, with BA you can create more complex itineraries through BA's interline agreements, which is all that Cruz said.
There will always be scenarios that cannot be accommodated. Indeed in your example, it would not have historically been possible to link separate PNRs where there was no interline agreement.