I would tend to support the view that CAX-SEN twice daily might be difficult commercially unless some form of regional support with marketing etc. could be obtained.
Having said that, there are some routes which you would beforehand reckon were surefire winners and they turn out to be failures and vice versa. CAX-SEN certainly has the advantage that road travel is not competitive so it's really all down to latent demand and rail competition factors.
The Stobart ownership of both CAX and SEN, plus its part ownership of Aer Arann, must tip the balance a little towards a positive outcome but nothing is going to happen any time soon that's for sure.