not just in 10 or 20 years, more like in 9 months!
The supreme court will probably rule that the Mietendeckel is unconstitutional in November, which will lead to a nasty situation for many renters with old contracts. If the tenants are lucky the landlord will just ask for the money "saved" to be paid back, but the unlucky ones will be evicted, as being in arrears of more than one month is grounds for termination regardless if you pay it back the the debt or not.
It's extremely unlikely that the court will strike down the law in a way that this situation arises. It's not like everybody starts to pretend that a law has never existed if it's ruled unconstitutional. Usually arrangements will be made for those that relied on it. Especially in a case like this, where another basic right is affected.
As I said, "strike down" usually doesn't mean "transport everyone into a parallel universe where the law has never existed but all people still have acted the same and deal with the fallout no matter how unjust". Rulings are usually much more nuanced than that (and constitutionally required to be that).
This is one more of those "software engineer tries to apply algorithmic thinking to law and gets ridiculous results" situations.
If you continue to be patient and repeat that humanity doesn't function according to algorithms, they'll eventually get the message or get bored and leave to work on the next disruption.
I have to say - I'm very jealous of my few friends who somehow managed to get a new place in the last few months. Their new contracts are already at the lowered prices so they don't have to fear the backrent like I do (and my reduction is quite big as I was paying way above the average for the area due to having a hard time finding anything in the first place).
The tenants are not in arrears. In the common case rents were lowered officially by landlords, with a footnote that the difference is due in full if the law is striked down.
And as stated in the third paragraph of your link, it's not grounds for termination if the landlord has agreed that part of the rent is suspended until the matter has gone through courts.
The supreme court will probably rule that the Mietendeckel is unconstitutional in November, which will lead to a nasty situation for many renters with old contracts. If the tenants are lucky the landlord will just ask for the money "saved" to be paid back, but the unlucky ones will be evicted, as being in arrears of more than one month is grounds for termination regardless if you pay it back the the debt or not.