Isn't this quite useless, when they don't have the copyright on the initial version, since they didn't require a CLA back then?

CLA allows them to relicense your contributions under their own license - e.g. proprietary

A DCO would be the more friendly option.

I think the question is, what use is adding a CLA if the core functionality was under (A)GPL? Unless you go back and get all the OG contributors to sign over their rights, how can you relicense?

Yeah, exactly that's my point. The role of Arduino is like that of a Distro, they own the packet repository and the packet manager, and maintain a build-system and an IDE. They aren't the initial copyright holder to basically any library. The only thing they really own is the Arduino API, but this is an API not an implementation. The compiler is GCC, the board specific methods come from the hardware vendor, the C lib is newlib or comes also from the hardware vendor. The flasher software comes from a different company.

The libraries are written by random people, what Arduino does is adopt them after ~4-6 years of existing, slapping a "© Arduino LLC" on top and maybe fixing the packet manifest. The role of Arduino is a vendor and maintainer, they don't really are upstream for much things.

I don't really understand how what they try to achieve with these new "terms and conditions" is legally possible. (https://news.ycombinator.com/item?id=45978802) They could release new software with different licenses, but they would need to rewrite most of the ecosystem to do that. Neither MIT, nor LGPL, nor GPL nor AGPL contain any reference to "terms and conditions" of one of the copyright holders, which should be followed on top of the license.

You can’t sign the CLA if you are contributing code which you don’t have the right to relicense and isn’t appear under the CLA