Either multiple different keychains or even you can have no keychain-like application in your system at all.

The WiFi passwords are usually stored in /etc/NetworkManager as plain files. Granted, they are not accessible directly by non-root users as they are being managed by the NetworkManager daemon, but there is nothing generic for such a thing. Signal rolling a similar daemon for itself would be an overkill. The big desktop environments (GNOME, KDE…) usually have their own keychain-like programs that the programs provided by these environments use, but that only solves this problem for the users of these specific environments.

To me it’s perfectly expected the Signal encryption keys are readable by my user account.

There is no single keychain on Linux, and supposedly on Windows too. Signal would need to either support a few dozens of password managers or require a specific one, both options terrible in their own way. This isn’t something that can be done without making broad assumptions about the user’s system.

Firstly, this is not “my argument”, this is EU’s argument.

Secondly, none of these platforms present it as a choice between paying and giving the kind of consent that by law needs to be optional and freely given.

Thirdly, being free to not use a service that is breaking the law does not make it any less illegal.

They can just charge €10/mo like every other company does, for example Netflix. They can’t offer it as an alternative to the “freely given consent”. It’s not freely given if the alternative is to pay to not give this consent.