There are (non-shady) firms that do exactly this for other areas (flight compensation, most notably).
There are some issues with contingency fees in German legal professional law. However, it can be argued that suing for these 5,000 EUR is just "collections", so it may be allowed.
The risk lies elsewhere: As I outlined in another comment, there is reason to believe that this may not stand on appeal, or at least that other courts in other parts of Germany may decide differently. As a result, it takes a lot of capital to keep all of these lawsuits going until the Federal Court of Justice or the ECJ have decided and there is legal certainty.
There are some issues with contingency fees in German legal professional law. However, it can be argued that suing for these 5,000 EUR is just "collections", so it may be allowed.
The risk lies elsewhere: As I outlined in another comment, there is reason to believe that this may not stand on appeal, or at least that other courts in other parts of Germany may decide differently. As a result, it takes a lot of capital to keep all of these lawsuits going until the Federal Court of Justice or the ECJ have decided and there is legal certainty.