I know quite a few young couples who are stuck in apartments and are desperate to buy houses with a view to starting a family. They're postponing this because their in negative equity. Most would have combined income of €100,000 - €120,000. They could easily afford a mortgage on a larger property and the absorption of the apartment's negative equity into that apartment. They just can't bridge the €50,000 - €80,000 to get them out of the negative equity problem. .
You can't have it both ways. If they can easily afford a bigger mortgage then they can easily either rent out their apartment or pay down the negative equity and then buy. Personally I find it incredible that people on 100K to 120K salaries can claim an inability move and hence an inability to afford a child. Can you explain how much is their current mortgage, the repayments, the size of their current apartment etc so that I can see it as clearly as you.
To put some context on it I have a sibling who has a very young kids and moved out of a vastly negative equity property in Dublin and is renting it out at below mortgage amount but is managing that and where they are living now on a much lower salary than you've quoted.
I'm not sure but I'm getting the impression that people want to move into houses with the click of a switch and take on even more debt while claiming they cannot do anything about the negative equity of the tiny apartment they have bought and would also like that to go away with the stroke of a wand.