I have. I bought tickets and they were very expensive. Im flying vancouver-sf round trip next month for 1/3 the price i paid for london-paris a few years back.
Doing the math again today, round trip paris-london on eurostar is 600$ in my money (320bp on eurostar site +fees +conversion to cad, leaving next monday). Vancouver-sf by air can he had for 275$ (corporate account, no visible discount).
I just checked the Eurostar prices. A ticket for London-Paris tomorrow and Paris-London on Wednesday can be had for $247. If you book a month ahead of time, it's below $50 one-way.
The airport in London and Paris aren't very convenient if you actually have to go to the city, and you need to add transportation costs to and from the city on both sides. Eurostar takes you to locations where people actually want to be.
AS for your 600 euros for a ticket, it's likely a last minute bought one at the most expensive time, it goes as low as 39 euros for a single, most people will pay under 300 for the round trip I guess...
Heathrow is pretty convenient to London with both tube and Heathrow Express service but train is generally more convenient than air if you're going to either downtown Paris or Brussels. Personally, I'd probably never fly those routes unless they were legs on a longer trip.
In general, I've found that in Europe there are pretty consistent large savings to booking rail well in advance, which is not nearly so much the case in the US. This can be the case with flying but air prices tend to be all over the map without an easily identifiable pattern.
The parent post was about French high speed rail. Your rebuttal was about a specialty link (the Eurostar) between London and Paris. That's 2 different countries and a massively expensive 35mi long undersea tunnel.
You're not wrong that French rail is relatively expensive, but I think a specialty link was not really a "normal" rail ticket to use to compare.
You can get a round-trip on the Eurostar for USD 140 in Economy, maybe USD 200 for popular departure times. That brings you right in the middle of the city, unlike a flight. There's a reason the Eurostar is popular.
If the Eurostar were typically that expensive, then its competition (both aviation and coaches/ferries) would be absolutely crushing it in the market, and they aren't, which suggests that that is unusual.
