What nation needed to capitulate for the previous military force authorizations against pirates on the high seas? Non-state actors are not a new development.
Personally I would say the AUMF should go away after the withdrawal from Afghanistan but let's not act like there's some magical international legal shield over AQ because they haven't taken over any other national governments.
You seem to be assuming that a declaration of war plays only into military terminology, but that's not the case. The AUMF gives the President Congress's blessing to use the military to defeat terrorist groups and their supporters (especially those responsible for 9/11), without giving the President the keys to the other aspects of national government which might rightly be involved in a war effort. So the executive has special wartime powers relating to the prosecution of the military effort within the constraints of current military capabilities (e.g. he can now mobilize some types of Reservists), but not other special wartime powers (e.g. no authority to call a draft, impose food rationing, etc.).
The Congress can implement their obligation to declare states of war however they see fit. The concept of the AUMF dates back essentially to 1798 so I can't see why we'd declare it illegitimate only now.
Let's see, an inconclusive pseudo war that ended with basically a negotiated discount to the original tribute demanded, but that probably cost a lot more than that tribute to actually fight.
You're right. We did set some kind of precedent and pattern there, though, of course, modern military power is capable of wasting ever so much more life and money on the way to an awkward inconclusive end.
But I don't think just because it was an old bad pseudo war that adds to legitimacy.
Personally I would say the AUMF should go away after the withdrawal from Afghanistan but let's not act like there's some magical international legal shield over AQ because they haven't taken over any other national governments.