So keep supporting it or give away how your stuff works underneath and allow someone else to. Bricking device into a landfill should not be decision corporation is allowed to take on most (all?) customer devices.
There is no excuse for not providing it from day one. And more than one reason to do it -- not only do you verify that you have what you need before they're gone, it allows people to find vulnerabilities sooner so they get patched before more people have the vulnerable device, and allows them to improve the software in general even when the hardware vendor is in a commodity market with margins to slim too do it themselves.
Setting that aside, I have yet to see a complex system that you can rebuild without a lot of undocumented know-how just by having the source code. Companies don't typically keep meticulous documentation on this, they keep people around to do it. When the product goes out of support, the people get transferred or fired.