I think the international connections from Amsterdam are as follows:
ICEs broadly every couple of hours from Amsterdam to Duisburg/Düsseldorf/Köln (and beyond) for the Ruhr area
ICs every couple of hours Amsterdam-Berlin
Thalys at least every couple of hours, hourly at times, to Brussels and Paris
3 (soon to be 4) Eurostar a day to Brussels and London
* The ICE services run 7-8 times a day to Frankfurt, with one train a day to Basel
* IC to Berlin runs every 2 hours, with some late/early trains only to/from Bad Bentheim or Hannover
* Thalys services are to be increased to an hourly service soon
* Eurostar 3-4 a day to London
* Nightjet services once a day to Zürich and Innsbruck/Vienna
* IC to Brussels runs hourly
In addition there are also several indirect international connections
* IC to Vlissingen (half-hourly) connects onto the hourly local service to Antwerp at Roosendaal
* IC to Maastricht (half-hourly) connects onto the hourly local service to Liège at Maastricht
* IC to Heerlen (half-hourly) connects to the hourly local service to Aachen at Heerlen
* IC to Nijmegen (half-hourly) connects to the hourly local service to Düsseldorf at Arnhem Centraal
* IC to Hengelo (once a day) connects to the hourly local service to Bielefeld at Hengelo
This seems a significant worsening in the service pattern and connectivity - why are these changes being made?
Additionally to what Trainbike46 mentions those changes are also part of so called "Programma Hoogfrequent Spoorvervoer" (Highly frequent rail transport programme). Those involve decomplicating infrastructure at stations enabling faster approach and departure and less conflicts between corridors. An example of such a decomplicated station is
Utrecht Centraal where trains are now able to enter the station at 80 km/h instead of 40 km/h, shortening journey times.
At Amsterdam Central the corridor Zaandam - Utrecht - Eindhoven will become conflict-free through the middle of the station, with to the south platforms where trains from Schiphol and Haarlem will terminate and to the north platforms where trains from Almere/Amersfoort will terminate. The international services (Thalys and Eurostar) would however conflict with that setup, as the yard where they are cleaned and maintained is beside the lines from Amersfoort and they have to continue to Schiphol. Therefore, those services will use the back-entrance of the Watergraafsmeer yard in the future to get to Amsterdam Zuid.
The changes to the services from Almere/Amersfoort are a result of the SAAL (Schiphol - Amsterdam - Almere - Lelystad) program, introducing a more frequent service there. In order to fit those services onto the existing infrastructure all IC-trains from Almere and Amersfoort will go to Amsterdam Zuid and Schiphol, eventually continuing via the HSL to Rotterdam (that's where the new ICNG trains will be used in later stages of introduction).
For Amersfoort that means for example 4 tph to Amsterdam Zuid instead of 2 to Zuid and 2 to Centraal (from practical experience: when taking the subway from the city center I'm always taking it to the wrong station to have a quick connection currently
). To Centraal more Sprinter services would run in future, also reducing the need for services to be passed by IC trains at Weesp.
Schiphol will get a highly frequent Airport Sprinter service instead of the current mix of Sprinter services and high speed services. The latter will continue towards Amersfoort/Almere as mentioned above, except for the international once which will terminate at Amsterdam Zuid.