There are few low-cost airlines operating between London and Paris or Brussels, particularly in view of the large size of the market between these places. I would contend that E* have been very successful in stopping the expansion of low cost airlines on those routes. It is not only the premium market that has been affectedIt has supported the development of one of the busiest roll-on-roll-off trade routes in the world, reduced costs and journey times and promoted a fantastically successful economic integration. All until 31 December 2020 obviously. Eurostar by contrast has... reduced the number of full service flights although notably not to zero, but done next to nothing to stop the expansion of low cost airlines.
I think that is a very blinkered view of the passengers travelling on E*, in my considerable experience of travelling with them. The markets which they do not target are those which they have no hope in competing in against low cost airlines, generally those involving travel to and from the provinces (of the UK or France) or further afield. No point in trying to conquer the unconquerable.Only in the premium air market though, and only for one part of England.
Eurostar's target market is almost entirely comprised of just three groups:
Business traffic on top price corporate fares
Very wealthy tourists visiting Europe, especially from East Asia and North America
Well-to-do London and Home Counties folk on their romantic weekend break in Paris, skiing in the Alps or taking their kids to Disney
The rest is mainly people who won't, for personal reasons, fly, staff on discount tickets, or enthusiasts
E* are a business, and consequently have to operate profitably. There is no way that a Manchester-Koln journey is going to be made by train at the same fare (or actually much cheaper than, due to the journey time penalty and changing) to compete with a low cost airline. The only way of your 'theory' being 'feasible' is if low cost airlines simply did not exist. Anything else is romantic myopia. No different to thinking that Ocean Liners are ever going to compete with Airlines for the London-New York market, even though it would be possible in theory and be entirely feasible to convey all the passengers on offer.Because it carries a trivially low number of people compared to the number who could, in theory, travel feasibly by rail between cities through the tunnel. I.e. next to nobody who is travelling from the UK outside the South East of England, next to nobody who is going to Germany, etc. That's simply the way that's been more profitable for them, while Manchester to Koln passengers go by air.
Being bitter about not being able to go on a through Manchester-Koln train for £40 each way is the stuff dreams are made of. There are some pretty seismic political decisions, on subsidies and immigration, that would have to be made first for this to be a possibility. Probably not a priority just now.