Earlier quoted context omitted.
The beauty of it is it doesn't matter to the bureaucrats: since the 19 years old can't live in the city he also can't vote in the municipal elections! It also has bad impact on the type of buildings that get built. If you have to build some rent-controlled housing as part of your development project, and some jurisdictions will force developers to do that, it pushes you to build the rest of your building to be as ups…
I don't know how it is in Germany, but in the US, rent controlled buildings can only have the rent increased if the landlord upgrades the building, so it actually incentivizes them to do upgrades, as that's the only way they can increase rent.
For example, in San Francisco, landlords can usually pass on some portion of the cost of capital improvements on to their tenants, but not all of it, and only until that portion is paid off, after which the rent reverts to the previous amount.
So yes, the landlord gets the improvement done at something of a discount, but still has to shell out cash, and for not much benefit since the rent will still revert to the previous level. So there's not much incentive to do it until after the tenant moves out, if at all.