Possibly weird idea: federal firmware escrow. The OEM gets to put a stamp on their product after submitting firmware source/keys to the FCC. When the OEM either declares the product not supported
or provides no updates for X length of time, the files are automatically published to a public repository. Perhaps there is an appropriate license which says essentially that it is almost public domain, with an exception (or fee?) to use it for any purpose other than supporting the lifetime of an existing product.
As others have stated, free software is one way of giving the public ability to keep things up to date but that's almost like the government saying people are allowed to clean up pollution. It doesn't put any pressure on companies to behave better.
Another issue is build-ability of open source code. If an OEM submits firmware source and keys to a third party, even regularly, who really knows whether it is actually functional and complete. Automated tests or sample hardware are possible ideas but have their own failure modes and could be difficult for to implement solely for this purpose.
Another weird idea for the above. If one requirement was deterministic builds, then in addition to source/keys, a suitable toolchain to build could also be required such that the repository stewards would only need to run exactly what the OEM provides, and if the checksums don't match then it means they are not in compliance.