The international platforms are not deleted! They were brought back into use from 2018-2019 to serve the Windsor Lines, which includes the service to Reading - platforms 20-24. That somewhat reduces the congestion at Waterloo; the station throat limits adding more services.

The extension to Euston was supposed to have 11 platforms. Even the reduced scope now being implemented is 6 platforms, I believe. All 11 were required to handle the eastern leg of HS2 [providing bypass capacity for the East Coast Main Line out of King's Cross and the Midland Main Line out of St Pancras], and services to Scotland and Manchester [bypassing the West Coast Main Line from Euston's classic platforms].