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Problem

Let’s say that you wrote a piece of software, and publish it under the GNU
General Public License, version 3.0, or the GNU Affero General Public License,
version 3.0 (for the sake of simplicity let’s just call them GPL because the
networking clause is irrelevant).

You would need to choose between two variants:

• GPL-3.0-only:
  Even when the Free Software Foundation publishes a newer version, your project
  will stay on GPL version 3.0. If you are the sole copyright holder, you could
  update to another version of the GPL (or, for that matter, any license) at your
  choice. If there are other copyright holders (such as contributors), you
  cannot unilaterally change the license, and would have to ask for permission
  from each copyright holder (or remove their code).

• GPL-3.0-or-later:
  When the Free Software Foundation publishes new versions of licenses, anyone
  could choose to use your program under the terms of the new license.

I find neither approach to be ideal. It is often impossible to gain consensus
of all copyright holders since some may be unreachable. I also wouldn’t trust
the FSF with the power to unilaterally change the license of my program.

Solution

Notice that in section 14,

If the Program specifies that a proxy can decide which future
versions of the GNU Affero General Public License can be used, that proxy’s
public statement of acceptance of a version permanently authorizes you
to choose that version for the Program.

So, in my projects, I simply write something like this:

This project is licensed under the GNU Affero General Public License, Version
3.0 only.

Pursuant to Section 14 of the GNU Affero General Public License, Version 3.0,
Runxi Yu is hereby designated as the proxy who is
authorized to issue a public statement accepting any future version of the
GNU Affero General Public License for use with this Program.

Therefore, notwithstanding the specification that this Program is licensed
under the GNU Affero General Public License, Version 3.0 only, a public
acceptance by the Designated Proxy of any subsequent version of the GNU
Affero General Public License shall permanently authorize the use of that
accepted version for this Program.

For the purposes of the Developer Certificate of Origin, the “open source
license” refers to the GNU Affero General Public License, Version 3.0, with
the above proxy designation pursuant to Section 14.

(… Standard DCO stuff …)

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.