Some might not know about it, but I think it's more likely that the problem is that the amount paid by the housing won't go up even if the rent does. As in, if they pay a certain sum regardless of what the rent is, one might be able to afford it when it's 400, but their budget will become too tight once the rent goes up to 500. Even if it's something like they pay half of the rent, the sum the person has left to pay will also go up and might not be affordable.
I had to move once simply because my roommate wanted to go live with her partner and I couldn't have afforded our (rented) place from that time on my own. I ended up from sharing 100 m2 with one person (plus a dog) to living alone in a space of 32 m2 with the same price. 