ok, just quickly change the entire layout of every station in Central Europe to have ticket control

Amsterdam Central Station already has this infrastructure for the train to the UK.

On exactly 1 platform out of 15 though, and it took several years to rework all the access routes to and from that platform. It is also already much too small for all the extra capacity it will need when Eurostar intends to start using longer trains.

>On exactly 1 platform out of 15 though

Given the relative frequencies of domestic trains vs. international sleeper trains I'm not sure this is a problem.

ideally yes but i don't think it can be done with the current organizational structure of the european rail network. not consistently, only bilaterally