To be fair, easyJet’s move from T3-T1 was some years after Bland sold GB. In fact, GB was one of the last bastions of international BA at MAN with BACON having been sold the previous year so I doubt it was to differentiate itself from the past. In actual fact, easyJet was offered T2 back then but ultimately Monarch made the move there and easy went to T1.
I get what businessair is saying in that the distinction between low cost/legacy/network has become blurred. For example, even easyJet offers connections, albeit not quite in the traditional sense.
Low cost is probably going to be the future of european short haul so carting them off to one part of the airport for the sake of uniformity might not make sense going forward.
You make good points about how the current crisis may hinder future redevelopment but if things start to resemble normality next summer who knows. What is certain is that T1 cannot continue for decades longer, it’s beyond sprucing up in many parts.