I don't think you're right.

The GPL 3 says:

You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:

...

d) Convey the object code by offering access from a designated place (gratis or for a charge), and offer equivalent access to the Corresponding Source in the same way through the same place at no further charge. You need not require recipients to copy the Corresponding Source along with the object code. If the place to copy the object code is a network server, the Corresponding Source may be on a different server (operated by you or a third party) that supports equivalent copying facilities, provided you maintain clear directions next to the object code saying where to find the Corresponding Source. Regardless of what server hosts the Corresponding Source, you remain obligated to ensure that it is available for as long as needed to satisfy these requirements.

So the GPL 3 explicitly allows anyone to sell anyone else's GPL 3 code. I've written code that I've published publicly under the GPL 3. Insodoing, I've granted everyone who can download a copy (which is basically anyone who can find it out there on the open internet) to sell copies of my code for a profit if they so choose. (So long as they also provide the source code in one of the acceptable ways outlined in the GPL 3.) I knew and understood this consequence when I chose the GPL 3. That is to say I granted everyone else the legal right to sell my GPL 3'd code on purpose.

Basically the only thing in the AGPL 3 that's not in the GPL 3 is:

13. Remote Network Interaction; Use with the GNU General Public License.

Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge, through some standard or customary means of facilitating copying of software. This Corresponding Source shall include the Corresponding Source for any work covered by version 3 of the GNU General Public License that is incorporated pursuant to the following paragraph.

So, let's say I make a fork of Mastodon that a) prevents people from completing the signup process until they've provided payment information and authorized me to charge them $1,000,000/day, b) charges anyone registered $1,000,000/day, and c) deactivates any account for which the daily $1,000,000/day charge failes to process or for which the user has cancelled their subscription.

And then let's say I set up a Mastodon instance with that source code and invite the public at large to join for a $1,000,000/day subscription fee.

If I did that, I'd have to make sure that my modified source code was available to anyone who "interacted with it remotely through a computer network... at no charge, through some standard or customary means of facilitating copying of software."

Now, does "interacing with it remotely through a computer network" include going to the home page (even if you don't ever subscribe or click the "login" or "register" button)? Prooooobbbbaaaabbbllyyyyyy? (IANAL and I doubt that's ever been tested in court, so I definitely can't be certain. I'd want visiting the home page without ever registering/subscribing/authenticating to qualify as "interacting with it remoteley through a computer network" for any software I put under the AGPL 3. (And I have published code under the AGPL 3 as well as other code under GPL 3.)) If so, I'd probably have to share the source code with anyone who visited the home page. But that still doesn't say anything about me being disallowed from charging a subscription fee.

As far as I can tell, the AGPL 3 says nothing to disallow anyone from charging subscription fees for any AGPL 3 software.

All that said, if anyone who ran a free instance decided to start charging a subscription fee, people could jump to other instances and still largely get the same content and interact with the same userbase thanks to Mastodon's Federation feature. So there are still practical reasons why Mastodon as a whole and also indiviual Mastodon instances probably wouldn't or couldn't start charging membership fees. At least not in an enshittifying imposed kind of way like Twitter is doing.