> I find neither approach to be ideal. It is often impossible to gain consensus of all copyright holders since some may be unreachable.

Well, licences are not universal wonder tools. They have restrictions about their use cases. But, narrowing this down solely to "GPL xyz" versus "GPL xyz - or later fancypants", I always found the variant WITHOUT the "or later" to be better. It simply adds more complexity when a licence can willy-nilly be changed, at a later time, when a change happens. I understand the use case for the "or later" part, as the GPL is very strict as well as an ideological tool against abuse from corporations (let's be honest here; and I think the GPL is a good licence, despite this too), but even then I find it better to stick to the simpler variants. It is one reason why I may use GPLv2. I also use MIT/BSD when I essentially don't care much. I don't think I have had a use case for GPLv3; and not for "or later" either. LGPL is also fine.

> It’s patently clear that the license allows this, and it surprises me that this is rarely brought up in debates about GPL-3.0-only and GPL-3.0-or-later.

I was unaware that a proxy can be designated upfront; so that's another complexity with regards to the "or later" part. What can proxies do? I dislike the "or later" clause; it really just makes this way more complicated than it should be.

The main advantage for using "or later" is not really to be OK when a new version of the license is published, as this happens rarely.

What you gain is the possibility of combining this code with any other code that is under a later version of the license. If there is code X under GPL-2.0-only and code Y under GPL-3.0-only, these cannot be combined, since each license declares that any derivative work has to be under the same license. If code X were under GPL-2.0-or-later, the combination would be compliant.

"It is often impossible to gain consensus of all copyright holders since some may be unreachable."

How one feels about that is a matter of where one stands. The GPL first and foremost protects the interests of software users. Not developers. Not companies.

In that regard, the above should be seen as a feature, not a bug. I believe it is the most effective way to protect the user from being locked-in.

I have long been skeptical of the "or later" clauses. I can imagine a not terribly distant future where RMS has passed away and GNU gets taken over by disinterested corporate psychos like happened to Mozilla, who then release GPL-4.0 without the copyleft, set up for industry looting of any GPL project that left in the "or later" clause.

With the "or later" version it's a concern that in the future someone nefarious could gain control of the FSF, and publish a GPL removing most of the copyleft provisions.

On the other hand, if Linux had used the "or later" version it could have helped prevent TiVoization.

It seems that "or later" would be putting an upper bound on the GPL restrictions? If additional restrictions are added, then users can still choose 3. If any restrictions are removed, the users can choose the later version.