Hi there,
I don't think that it is the reliability per se that is the issue. Planes are, as with all mechanical equipment, liable to problems from time to time. Let's additionally face facts, whilst when it happens and our flights get cancelled or delayed everyone is annoyed but the reality is if there is any hint of an issue we would all rather be safe than take the risk.
I thought that the issue was that the current L/H schedule needs ideally 2 more 747-400's to run so whereas previously a substitution frame could be found quickly, in particular for LHR outbound sectors, this is now not possible as there is no slack in the system (I have had this once with an LHR-HKG where a substitution was made within 1 hour!) Additionally some of the very minor stuff that was possible with 1-2 airframes sitting around at base at LHR which sometimes can spot something has suffered (this is not the routine standard maintenance schedule which must be adhered to strictly) and this may be increasing the problem.
As I have stated on here previously the key will be the rate at which airframes are retired as the new birds start to become part of the L/H fleet (nice though they are on the S/H fleet in terms of operational advantages its nil). The pre-BMI plan I understood was pretty much 1 new frame in and 1 747-400 out but I really have my doubts that this will occur. Of course it will depend on the D check dates of frames my understanding is a D check is basically rebuilding the plane (all but) so it won't be worth doing this to then retire one of the old birds 6 months later....
We shall see as I suspect soon we will see some sort of announcement regarding the first couple of retirements....
FD.