Very few people will be happy to take an 8 hour flight with no refreshments and a small underseat bag only. Once you add all the extras to "low cost" flights usually full-service is cheaper. Yes, some will be happy with that, but they'll be few enough in number to make it uneconomic. Just about the only way you can save is by cramming seats in a bit e.g. 3-4-3 777s and 3-3-3 787s.
Au contraire.
For the right price, people would.
Not too long ago people said the same about Europe. How can go on holiday without a checked bag? Who can fly without a meal?
But, suddenly, now people can do these things.
Even BA has 5+ hour flights now where that's all you get is a tiny packet of crisps and a small bottle of water along with hand luggage. The market has moved on and that's what it accepts now.
The same could easily happen with long haul.
Nobody has quite made it work in Europe quite yet, but the model has been applied profitably in Asia with Cebu Pacific, Air Asia X, Zipair, Scoot among others. And these airlines range from startup challengers like Cebu and established national airlines like Scoot (owned by Singapore) and Zip (owned by JAL Japan Air Lines).
I certainly wouldn't say it would be easy. Plenty of short-haul European LCCs have gone bust. And it's one of the reasons you have to admire Michael O'Leary. But I don't think the EU-Americas market is so special that LCCs wouldn't work on it.
For me, I think Cebu's model probably would be the best application. Get those planes flying to major NE cities and to sun destinations. Though that's just my feeling.
Ryanair is constrained by the 737, introducing any other aircraft would be an operational nightmare and the margins are just too small for it to be worth it to introduce another fleet.
No doubt they've looked at it and thought better of it.
Though they aren't constrained by the 737.
For one, they already operate the A320. Plus they'll soon operate the Max 10s which will require five cabin crew instead of 4.
Any long-haul operation would likely see a new type added.