The Dutch Parliament has called on the government to end all air service between Amsterdam and Brussels to help the environment and also to free up needed slots at Schiphol Airport for longer-distance routes, and the Belgian government is in agreement.
Train time between the two cities is only about an hour longer than a flight, and may actually be faster when you eliminate city-to-airport transfers and security lines.
It's not clear yet whether the two governments will take final action, and there may be opposition from the airlines as well. Previous attempts to shift regional flights from Schiphol have failed.
At present, Schiphol is under restrictions that don't allow it to add any more slots for flights at least until next year, and possibly beyond; it can't expand, and an attempt to create a second field at Lelyveld met heavy opposition and is off the table at this point.
A complicating factor may be who actually flies between the two cities. Since Schiphol is one of Europe's major transfer points for intercontinental travel, a large part of the Amsterdam-to-Brussels traffic may be passengers arriving in Amsterdam from distant points and then continuing to Brussels. The Dutch government is looking into more coordination between its railroad and the airlines.