You're being pretty uncharitable in assuming that there would need to be exactly one protection mechanism and that every source would necessarily have that same protection applied to it.

There are a spectrum of solutions to creating anonymity. If your source is just a faceless worker who happened to overhear something about their boss, then all they need from you is a burner phone, an email address on a public server, and GPG keys—all of which you can give out the public halves of.

If your source is Edward Snowden, and his information is about the state, then I would suggest that there's really no way to do things other than full-on "witness protection" in the sense Snowden achieved: the state is going to figure out who leaked the information, so even without giving other journalists access to your source, anonymity for them (where they currently live) is no longer possible. You may as well fly them to Russia; at which point they're safe enough that you can just give them the a public email address, tell them to access it from an Internet cafe over Tor, and now they've once again got a contact point that anyone else could interact with them through without "blowing their cover." So why not give it out?