Conduit is also licensed under Apache 2.0, so it could also be taken closed source at any point in time. The reason this wouldn't impact Conduit as much is that there're other contributors, whilst Synapse and Dendrite are almost exclusively developed by Element.
The CLA is necessary since Element funds the development of their servers by contracting with companies, governments and institutions which have special needs. Publishing those patches might be against their customers wishes.
The AGPL ensures no one else can make proprietary changes but Element because of their CLA. This makes it unattractive for companies and volunteers to contribute to Element's servers, which isn't a problem because those contributors didn't exist in the first place.
As I understand it, the people who feel strongly about this change feel like their trust was betrayed by Element. The others are probably corporation's like reddit who don't want to contribute anyway but are now not able to profit off of Elements work.
My opinion is split. On the one hand I like the change to AGPL, since it forces forks to continue to be foss. On the other hand, Element continues to be allowed to license the code differently, so it doesn't really change that the code could be closed off at any point in time.
The most important question is whether this change will benefit Element. Status quo is companies taking without giving back. Now corporations and volunteers won't contribute code because of the CLA and AGPL. This means Element hopes those corporations will contract with Element to get access to differently licensed code for a monetary contribution.
I think reddit will just develop their own server, but maybe smaller companies (like in the health care sector) will pay Element.
Beeper's software stack is partly open source. They develop the mautrix bridges for WhatsApp, Signal, Discord and many more as open source.
What they keep closed source is their clients (and probably infrastructure).
The documentation to host their bridges is great [1] and the bridges works with most server implementaions (I know of Synapse, Dendrite & Conduit).
I'm running signal and whatsapp bridges for years now without problems, but it's definitely simpler to pay for Beeper.
The problem with these bridges is that they have to decrypt messages and encrypt them again with matrix. This means anyone who controls the server running bridges has access to your messages. Self-hosting means I'm still in control of my data.
[1] https://docs.mau.fi/bridges/index.html