Yes, Waterloo International was meant as long-term terminal for Eurostar services when built. When the channel tunnel was authorised and built, the UK government did not sanction a high-speed railway to a London terminal. This is why the Class 373 Eurostar trains were built and equipped for 750V dc third rail operation, in order to use the existing railways to reach the new terminal at Waterloo. The decision meant a slower journey to the channel tunnel in the UK, which was always a bone of contention with France and Belgium who provided links to their high speed railways direct to the tunnel. And why 25kV was never envisaged for Waterloo International....
After only a few year's of operation it became clear that trains speeding through France and Belgium and the tunnel, then taking a slow and scenic trip through Kent, was a major embarrassment to the UK and eventually the government of the day sanctioned the building of the high-speed rail line (HS1) and new terminal at London St Pancras. A compromise was that the line had also to take a Kent commuter train service and also provide paths for freight direct into London.
So, with HS1 built, Waterloo International became a bit of an embarrassing unused station. There was no forward plan for its reuse, as closure was not envisaged. It is only now that, along with other major changes to the "old" Waterloo station, that Waterloo International is at last being integrated into Waterloo Main station.......
It is a sad (but common) story of bad political decisions and influences on the railway. There were many more associated with the channel tunnel - such as regional Eurostar services and sleeper trains between the UK and the continent.
You have probably poked the ant's nest here, to use a colloquialism....