If any ship deserves GBTT, its QM2...
Agreed...but for her to get it, would have required QE2 to forfeit it in 2003...which would have been a sort of Catch-22 from 2003-200X.
I'd much rather see a code with a 75-year direct Cunard history attached to QE, a Cunarder, than some bulker or tanker that gets the luck of the draw in the radio code lottery.
Of course...if we were REALLY going for linearity (and were to set aside the name debate)...both QE2 and QE(II) would be assigned GBSS, QE(I)'s code, which seems unassigned ca.2010. QM2 would be GBTT, in the footsteps of QM. Or at the least, QM2 would have adopted GBSS. (This is the sort of thing that keeps my OCD up late.)