It's interesting to read this and note that most discussions seem to be based on leisure travel. However, when I use E*, most of the passengers are either (a) leisure or semi-business [mixing business& leisure] between the London & Paris areas, or (b) business like me, travelling a bit further one or both ends.
I have found that E* has been, for journeys not originating near a London airport, faster, cheaper or comparable, and more convenient than air travel.
For comparison purposes, I'm satrting from Newark on the ECML, and travelling to Belgian stations or stations within 2-3 hours of Paris GdN.
Check in is no question easier: to be safe, allow 2 hours at airports, 45 mins at E*. And arrival with bags is much faster on E*, just walk out, no waiting 30 minutes or more for bags to be unloaded.
Transfers are the main saving: It takes up to an hour to get to/from London or Paris centres from Heathrow, Gatwick, Stansted, or Paris CDG. A bit less for Brussels airport (not including Brussels Charleroi: it's miles away, which is where Ryanair totally fail in many cities!). St Pancras and Gare du Nord are right there, ready to take the metro/tube to another station if required. And Brussels Midi is no more than a cross-platform change to go almost anywhere in Belgium.
Ticketing such journeys is something of a pain. For best value, flexibility and safety of connection outside London, you often need the CIV tickets that are very badly advertised - e.g. ewark to London Anytime E* CIV was about £34 last year, compared to the normal £74.50 Anytime. Hard to buy unless you go to ticket offices though. The even cheaper Advance CIV still maintain the guarantee of connetion if E* is delayed, so a no-brainer, but it does mean buying 2 or 3 tickets for the full journey, which is undoubtedly complicated, and it's almost a surprise no agency can make the case for selling them as packages making money from the commissions. But in this fractured open market of travel, airlines are certainly even worse [recent experience of KLM customer care reinforced this opinion!], no chance of guaranteed connections if another operator delays you inbound, so i'll take what i can with the CIV system, thank you. Want to know more about international rail? Search the "Man in Seat Sixty-one" ! - no connection to me, but a really helpful!
My other complaint would be the timetabling, especially now the new trains run less often, and the way the earlier morning trains don't connect well out of arrivals at StPancras & KingsX. Just a bit too early to make it through security unless you sart the jorney into London unfeasibly early, although to be fair the airlines have no hope there because of the check-in/security times.
You can't be rude about the security, it is faster than most airports, and is forced on E* (who'd choose that complexity after all?). You can see how it's been crow-barred into the facilities at each terminal!
Wasted opportunity? NO. Is there more opportunity? Well, maybe, but it's not easy. You could take a bus approach and maximise ridership, but whether profit woudl be there is questionable with so many fixed costs. A simpler pricing approach? - yes, just like the rest of the transport sector, it's wanted by customers but will only happen with strict international regulation, which is not going to happen any time soon. And of course, we'll ahve to see how the extra destinations develop in years to come. That might be the route to the desired growth!