Thank you for your detailed reply.
> it is (theoretically) trivially easy to work around - just contract out your modifications, have the contractor keep the copyright and act as the only user of the software with an indefinite license
Would this not require the contractor to comply with section 13? At this point, you would be paying the contractor for ensuring legal compliance instead of doing this yourself, which is something that companies sometimes do anyway.
> AGPL doesn't actually say how compliance with section 13 must be achieved... MIDI is technically a network protocol
MIDI is indeed a network protocol, but I wouldn't read section 13 as requiring that the network used for the offer of source code or the source code itself is the only network used by the software. If you had a remote MIDI sequencer that users could connect their musical instrument to, it would almost certainly have something like a Web interface as well (MIDI isn't by itself an internet protocol, which I'm sure you already know) where the source code could be offered.
Ambiguities inherent to the diversity of software don't negate the general legal principle of good faith. Posting a link to a repository on GitHub with your modifications is usually sufficient for compliance, and edge cases like where GitHub is down or blocked for your user can be resolved with good-faith email correspondence. Given how much fuzziness there is in law, I don't think it would help much to be more prescriptive in say, requiring the offer to be made over HTTP.
> or-later is generally a bad clause for any license, since it hands over all licensing grants to whatever entity you trusted to write the license
This is true, but I consider it to be the lesser of two evils. The evil that or-later avoids is that of licence incompatibilities and ambiguities, which are frequently complicated by a difficulty in tracing the authorship of older software. Those programs which were licensed with an or-later clause can typically be 'rescued' by a new revision published by the licence steward, similar to how the FSF carved out an exception to the GFDL for Wikimedia as discussed elsewhere in this thread. The evil that or-later causes, that the licence steward makes a compromising change, is quite limited: since they can't restrict rights beyond the older licence, the worst-case scenario is that they go against the spirit of copyleft and make it a fully permissible licence, which is still 100% FOSS and in practice the FSF are not going to do any time soon.
Again, thanks for going into depth. I agree that there are improvements to be made, and I certainly wouldn't consider the AGPL-3.0-or-later to be the 'be all and end all' of FOSS licensing, but I'm not yet convinced it's a bad choice! :)