Not trying to dig this one up again but the reasons have been explained time and time again. And... What makes you think 67s will be more reliable? As proved by both the DBS examples in Scotland - where west coasts 37s have ended up working the sleeper, or DBS using 2 to work it, ATW - Non ETH 003 for example, or on Chiltern where failures with the locos where common. Now with the 68s, the few and far failures have been with the stock...
67s arnt the wonder loco people seem to think they are - 57s may be be a mixed bag of a fleet, when they work they work. But when one has a failure, because there's so few of them it gets reported more.