Ah, first, we both (I assume based on your use of "old") made the common mistake of confusing the Deckel and the Bremse. So to clarify, I am assuming you meant "the old Mietendeckel" and that is what the court struck down - the Mietpreisbremse is still in effect. If this assumption is wrong and you rather did mean a pre-2015 version of the Mietpreisbremse implementation, sorry.
> The issue with the [Mietendeckel] was modifying existing ownership of land and buildings.
Was it? My understanding was that the decision ended purely on jurisdictional grounds. (Quick summary for anyone following along from the US, Germany has a kind of "inverse 10th Amendment" - the states can regulate things only as long as the federal level does not.)
https://www.bundesverfassungsgericht.de/SharedDocs/Pressemit...
By passing §§ 556 to 561 BGB, the federal legislator conclusively exercised its concurrent legislative powers extending to the law governing rental fees as part of civil law... the Berlin Rent Cap Act constricts the latitude afforded by federal law to the parties to a tenancy contract and introduces parallel legislation governing rent at Land level that contains statistical and not market-dependent stipulations.
In other words, I thought the problem was only that the law intruded on a legal framework between private housing providers and the federal government, not any particular aspect of what it changed.