I googled around a bit for legal cases and for advice aimed at Dutch companies, but because the law is so recent there's not much. My hunch (as someone who is perfectly clueless about legal matters) is that a restaurant or a shop might find it hard to argue that replacing one full-time worker with two part-time workers is impossible (given that the work is already organized into shifts) but perhaps companies that employ knowledge workers could successfully argue that you can't hire two part-time coders to do the work of one full-timer because the additional coordination costs in terms of additional meetings, additional equipment (i.e. laptop), perhaps the need for additional management personnel, and so on, are an unreasonable demand on the company. I could also see demands to reduce work to 4/5 be rejected when few people at work share your job description, because finding another employee willing to work 1/5 is really hard.

There's also the power dynamic to think of: are you really going to ask your boss to change your hours if you think that he/she will react badly and only grudgingly allow it?

Or maybe I'm completely off base... but anyhow, I guess I just wanted to point out that it's not clear to me whether this law has teeth.