Talking to Swissport dispatcher who had offloaded a 738 that had done CUN-LGW with a tech stop, which he said was because of low load factor on the route. It sounded unbelievable but I thought I'd ask the Q..
I can confirm that this is untrue. However, I guess the dispatcher just got it wrong. The 757-200 has been used as a last resort cover aircraft for long-haul flights. This has happened due to operational/technical reasons in the past but nothing for a while.