I think it's rather clunky having some of those callsigns, as they are pretty hard to say. I think BA are quite lucky as they use the flight number where possible whereas Lufthansa use trigraphs. If everyone used the new system it would just be even more confusing, especially as Lufty change the callsigns every season. Intuitively I always thing they're "Fraction" rather than Lufty with those callsigns.
easyJet use the new system for UK domestic and LTN, but not LGW and STN.
I remember the chaos back in 1995 when BMA1 became BMA1NL, much less elegant but avoided the problem of
BMA1 GLA-LHR
BMA51 EDI-LHR
BMA81 BFS-LHR
BMA331 MME-LHR
BMA411 LBA-LHR all working Heathrow Director at the same time....