The "problem" is that a rent controlled apartment is some weird chimera of ownership and renting.
You could take the difference between the market rent and contract rent to calculate a subsidy. Then you could calculate the net present value of the future subsidy payments. This is often a very significant amount of money that you just lose if you move.
Another way of looking at it. Let's say you need 40sqm of space. You rent an apartment of 100sqm. These square meters don't bring you any joy of quality of life at all. If you owned you would move. If you rented at market rate you would move. But if you rent it at a controlled rent you must hold onto it for dear life because maybe you will need it when you start a family in the far future. Getting another contract this good could be very uncertain.
You could take the difference between the market rent and contract rent to calculate a subsidy. Then you could calculate the net present value of the future subsidy payments. This is often a very significant amount of money that you just lose if you move.
Another way of looking at it. Let's say you need 40sqm of space. You rent an apartment of 100sqm. These square meters don't bring you any joy of quality of life at all. If you owned you would move. If you rented at market rate you would move. But if you rent it at a controlled rent you must hold onto it for dear life because maybe you will need it when you start a family in the far future. Getting another contract this good could be very uncertain.