I've not been on Eurostar for nearly two years now, so perhaps things have got worse, but even though there are queues for different things - they're not anything like the same as flying.
Arrive at St Pancras, maybe a short queue to get through the ticket gate. Then a queue for security, which still takes just a couple of minutes in all of my travels.
Then an equally short queue for French passport, and British (if they are there, which is rare).
When it comes to boarding, no queue at all unless you really want to.
Adding in another queue upon arrival back in the UK for passport checking shouldn't much hardship, as long as the queues aren't too long - and that's a manageable problem.
For the convenience of getting on the train, finding my seat and getting out the laptop and relaxing, knowing there's no safety briefing, no need to switch things off or stow them away for take off, or landing... No, if a train is a viable option, I'll pick it over flying almost every time.
Once trains go beyond Brussels and Paris, I am sure a lot of people will be picking the train over flying. Some people will need the (perceived) speed of flying, and some people will be close to an airport which gives it an advantage, but many others will realise the benefits of being more productive on the time spent on the train. Rather than merely travelling, you could be working the whole time - with phone/Internet working without ridiculous charges* etc.
* I accept data roaming is a rip off now, but there are ways to reduce costs and I hope the newer trains will have Wi-Fi anyway.