It is ridiculous. Eurostar don't have to operate the pricing policy they do, it has nothing to do with the physical infrastructure. They do have to have compulsory reservations, but plenty of trains the world over have those but the fare for each seat is fixed without any yield management.
No, it didn't. The ferries to e.g. Oostende still existed until about 2000, it was the airlines that killed them, not E*.
Anyway, who went to Paris at short notice when it took 8 hours?
In the past, I've gone by train/ferry to Paris at just hours' notice; and also Brussels-London and back via Ostend at a couple of days' notice, to pop back for a meeting in London while staying in Brussels. The (then) relatively seamless journey, combined with no short-notice price penalty, made that feasible and practical. There are times when I'd find it convenient to still be able to do that if the train-boat link, with through tickets, still functioned.
To a hypermarket for wine on a train? It's only worth it if you buy more than most would carry.
If it was a viable market, though, the coaches would already be doing it via the tunnel. Connex tried it but it didn't really work.
During the years when the Cheap Day Return London-Calais tickets existed, I made many trips over for the day - for all sorts of reasons (including shopping, on occasions; sometimes just for a "different" day out). Sometimes I took my bike, which speeded things up to/from the station at each end, and (in those days) getting on and off the boat too - as well as getting into town at the French end of course. (You can get quite a bit of shopping back if you have a couple of big panniers and a bike rack.
If this option was still available, I'd certainly still do the trip from time to time, and I know others who would too.
Agree completely on Dieppe - you can do some very nice day trip/weekend breaks in that part of Normandy.
Yes - Dieppe is now the best route for rail-sea London-Paris (and it always was a useful alternative to Dover-Calais); though the longer crossing makes it not viable for a day trip.
The real bugbear now is London-Brussels, which has no feasible rail-boat connections at all, despite the integrated port-station at Ostend.