That seems to be an unrelated discussion. There is no discussion whatsoever that bicycles would be taxed in Belgium. Bicycles have been in use for more than a hundred years at this point and no one suggests taxing them.

There is a clear cut off to distinguish bicycles (and wheelchairs, etc) from other vehicles. Any "pedal assist" that stops assisting when the bike reaches 25km/h is a bicycle, tax free and insurance is optional. The bikes with stronger motors going beyond this or that drive without pedaling need already for many years a license plate and insurance. Not sure it's universal but very common in Europea, probably based on some EU standard document.

Note also that already now everyone's taxes pay for all roads from motorway to bicycle bridge and sidewalk. The income from road taxes for cars and other motor vehicles is tiny compared to the infrastructure costs involved.