Whether or not you call that success or failure depends on what you're trying to achieve, but usually these sorts of actions are trying to create housing affordability. Taking most of the supply off the market for new renters (by selling to people who can afford to buy and by incentivizing those who were already renting to stay for as long as possible) definitely does not create an affordable situation for those who need it.
Without creating more supply, you end up with one of two problems for folks at the bottom. Either you don't regulate housing, and their prices go up too much for many to afford, or you do something like this and you keep the existing stock affordable but guarantee that there won't be nearly enough stock for all the people (at the bottom of the economic ladder, at least) who want it.
In this case, you're not helping poorer folks as a class, you're only helping the specific set of poorer folks who were already renters (and who don't need to move into a bigger place, since they can't leave their current rentals). If you want to move to Berlin and you're not rich, you're SOL.
That certainly doesn't seem to be the intended effect, so based on that I don't think it's fair to call it a massive success.