Most likely the same as what I would do when Lenovo stops making spare boards for the model I have and I can no longer find any on eBay - bin it.
Except in the case of Framework going under, it's much easier for another manufacturer to release a compatible motherboard since the form factor is published and fairly simple. Similar to why it's easy to replace my desktop's motherboard even when the original one is no longer made. If anything, the current OEM making the motherboards could keep making replacements on the same tooling after Framework is gone, so long as there are people buying them.