The problem with it is that the conversion efficiency of a meat -> meat-eater -> meat step is painfully inefficient.
The alternatives would be, if one won't eat plankton personally, farming something that does.
As for low value feed, there are a couple of issues at play: one is that a major reason why fishermen are bothering with low-value catch is that all the high value stocks have already been pushed into sufficient scarcity that they can't support a fishery anymore. The other one is that many low-value fish can be slurrified into Surimi just fine, which is a rather more efficient use of the protein than feeding it to a shrimp and then eating the shrimp. Not as classy, admittedly; but better than plankton and more efficient than authenticity.
RE: regulation and enforcement, I don't disagree with you there, I'm just inclined to take the position that (thanks in part to the 'international' status of large parts of the ocean, and in part to good old corruption, weak enforcement, and the like) expecting 'regulation and enforcement' to actually happen in time is not a safe bet. It's a good goal, and should be pursued; but the awfulness of the current state of affairs can scarcely be exaggerated.