Then why do Tom still use 767 for TFS in winter, also LPA ACE and let alone 757s there everywhere. In summer the 767s are king still on the PMI, IBZ, CFU etc. it would take almost 2 738s to replace two 326 config 763s on these routes. I think an all 738 for short medium hall was the wrong way to go, the major charter destinations could take a wide bodied aircraft, why send two aircraft, two flightcrew, 2 sets of cabin crew when you can stick over 300 in a 767 send it somewhere easy to fill MAN-TFS or LPA in winter would be an example.
They use the 757s simply because they have them and will do for the next few years untill the leases start coming to an end... Yes they are a great aircraft and have done Thomson (and their predecessors) proud. But, as Cabincrewe says things have changed and will continue to do so. It's not easy being able to fill those 757 aircraft year round to make them economical when compared to the 738. Yes it's great for the extra capacity in the peak summer months and perhaps the winter Canaries but unfortunately in this day much of this extra capacity is filled up using highly discounted holiday prices...
People also require more flexibility with their holiday stays now... This is why more frequency per week rather than capacity per flight is the way Thomson and the likes have been heading for a while now. For example, at many of their regional airports you can choose a day to fly to PMI atleast 6 out of the 7 days a week during the summer. This is just one example of the changing needs from the customer to which airlines have had to respond - thus needing smaller more economical aircraft.