Understandable, but I do feel the world still needs some loco-hauled passenger trains left as they are declining worldwide. I like loco hauled trains since you don't feel motorized bogie vibrations in the carriage.
It may be said that they still are used extensively in the continent. DB uses them in their regional services.
Not all EMU carriages are motored; 745 has only 4 motorised bogies in a 12 car train, effectively a top & tailed loco hauled set (well, half a loco at each end, sandwiching a loco right in the middle).
I don't mind too much either way, but can see loco-hauled stock used for demand-responsive services. Where the platform length is in no danger of being exceeded, I see no harm in having a loco.
Returning to topic, I much prefer the Mk3s. I think they are also slightly longer than the Mk5as, and have been proven to be much less problematic to operate. I do think that to the extent that Chiltern is not competing on speed, a locomotive-hauled & comfortable offering may be a more appropriate package. It may well be more cost-effective as well: the IC225 was much cheaper per seat than the TGV and ICE, and with actual 26m carriages (Mk6), this could be accentuated further. Have a simple loco with tried and tested technology.