I think EWS did play a big part in the loss of mail on rail. I don't think it helped that they bought totally unsuitable locos for the service, i.e the 67s. They were based on providing a 125mph (where line speed permitted) network but were too heavy to run at that speed so provided little benefit but increased costs (purchase price had to come from somewhere).
Another big factor was Hatfield, which caused serious delays to rail in general, so annoyed RM and made air and road far more attractive (both might suffer occasional delays, but not delays for weeks).
The one I often wonder is if there is a market for International Mail, especially if as others have said other carriers are included. I think it would be nice to see International/Long Distance Domestic high speed containerised freight in general, competing with both air and road.
I think it could very much be a market for a UK compatable Traxx. Loco haulage would be superior to units because of the flexability (trains could be made up to length required, instead of being fixed formation or having whole units sitting around), simplicity of stock (everything below the floor leaving floor up as 100% cargo carrying space, plus individual vehicles much cheaper to build helping the flexability) and the fact that if a train is split on route then local locos can haul portions reducing staff training costs and reducing complexity (especially if portions go off the core high speed network).