The thing here is that the RF wasn't very big: Baudouin had no children, Josephine Charlotte went to Luxembourg, so all the descendants came from Albert, he only had 3 children so there wasn't much talk about dotations and roles when his brother was still alive, specially since Philippe and Laurent took a long time to marry.
Now Albert has TWELVE grandchildren, Philippe alone has FOUR children, of course the RF had to be downsized again and the King's siblings should earn dotations based on what they do, it's no obligation of the State to support them and Albert couldn't have possibly thought that he was going to get the same amount of money (or close) when he was reigning monarch, he doesn't have a huge household to pay for or has to pay the bills in the Laeken complex, doesn't have children to support, etc.
Philippe is doing what he should IMO and it is very sad that his family is turning their backs on him, Mathilde and his children because of it.