How about we halve the billionaires while we are at it?

Treating the symptom and not the problem IMO.

Invest in worker co-ops that are equally owned by the workers instead.

Yeah, but it not unlikely that “Die Linke” wont get into Parliament because of the “5% Barrier”(A Mechanism that prevents parties from entering the German Parliament, if they are below 5% total votes. There are some complicated exceptions, but basicly this is it). Furthermore many Parties dont want to form a coalation with them and this could be against german constitution … So this is very unlikely to happen.