Having now witnessed the entire operation of shutting the doors, clearing the platform, having a whole troop of Securirail personnel guarding the platform for the duration of the train's 20+ minute stop and then opening back up again afterwards, I am not convinced about the sustainability of this to be honest.
First they shut all the roller doors on the platform, then the south-facing platform 3/4 escalator from the Thalys waiting area downstairs is shut off with another roller door, then they open some sliding doors coming from the Eurostar terminal and (in theory) the passengers will go up from there. It appears the same might happen in the south corridor end, although building work there is still ongoing. There is a lift that is normally available to both sides of the island platform, but that was only available to access from platform 4 (so an issue for disabled Eurostar passengers to access platform 3?).
While all this was going on a Thalys rolled in to platform 4 and disgorged hundreds of passengers. The tight squeeze as they all headed along the platform to the available escalator at the northern end was a problem and should there be any kind of emergency issue this will be a real choke point.
It seems like the operation needs a lot more security and railway personnel. That begs the question of are they recruiting more just for this, or are they borrowed from other duties around the station? Does it leave the rest of the station vulnerable each time a through-Eurostar rolls in?
All this hassle could and should have been avoided by having the passport controls on the London arrivals side, like a plane...
As for viability of other destinations: DB showed that the claimed rules around needing continuous 400 metre trains were a nonsense. Eurostar could have changed their follow-on order with Siemens to 2x 200 metre sets to split in Brussels with half to/from Amsterdam and half to/from Germany. Maybe they still can with a new batch.