This organ was built to reproduce as closely as possible a certain type of Baroque instrument, and through the research and experimentation necessary for the design and building, to understand the Baroque organ better. Such instruments were entirely adequate - in fact superb - for playing the music of their time, and the virtual impossibility of changing stops while playing is a clear hint that the music was not intended to undergo changes in registration. It must also teach players how to perform the music in an appropriate manner.
If players find it necessary to change stops during a Bach or Buxtehude fugue that surely means that their instrument lacks any registration worth hearing for very long.
Of course, with the aid of registrants, many Baroque instruments can be surprisingly versatile, but there can be a temptation to tweak an instrument to make it more versatile, as was done to the Muller organ in the Bavokerk in Haarlem, which no longer sounds like a Muller.