@rysiek There's an answer on that website and it's very wish-y-wash-y unfortuantely.

The example is someone in Belgium owning 1.25M€ in addition to a house being considered "ultra-rich".

That's an interesting data point but I suspect that is *way* too low a barrier for many people.
Exceeding 1.25M€ is easily done by inheriting a house that was worth 50.000€ when it was built 60 years ago.

And people who are looking at potentially receiving at such an inheritance are going to go "hmmm, that might hit me too. Better not sign!". And I can totally understand that because in general everybody is always looking out for themselves first.

I do not understand why these initiatives are not putting out a statement such as "we consider people having liquidity of more than 5M€ to be ultra-rich and are interested in taxing these!".
That's a clear statement and the cut-off is far enough removed from the possibly-rich-enough-after-inheriting to not alienate that big part of the population.

Instead we get these non-answers covered in relative terms resulting in a lot of people going "yeah, thank. not signing...".