Would 390s not be likely to attract new passengers to the network? Personally I'm no fan of these trains but they do seem very popular with the general public on the West Coast.
Why would they? The reason they were popular on the West Coast is because they replaced stock which was largely falling to bits, the same cannot be said about the 225's. They also appeared around the same time as the entire line was thoroughly modernised - whereas the ECML is in nothing like the state the WCML was.
Most people wouldnt even notice the difference.
I doubt it's the 390 that attracts poeple anyway - they want a clean, modern train with seating and reasonable fares. Virgn often offer this, it's no wonder they are popular.