E.g. what we could do was sending Bitcoin to a "n out of 2n) signature address. And then invest the money via majority vote.

The disadventage would be that the invest side isn't decentralized (arguably it isn't either in the Eth ecosystem - people make foundations for this etc. It would also make it pretty hard for individuals to pull out their money early.

Finally there is the question why we would want DAOs on Bitcoin? What's the advantage of decentralization here? What's wrong with centralized stock markets and buying shares? Decentralization just for the sake of decentralization doesn't make any sense.