I see you confuse source being available with being alive and actively developed. Sure, in theory, with source available its possible that someone pick it up and keep it alive. But often you either need a critical mass for that. MySQL may go on forever, but I can walk through the GitHub or SourceForge cemetery to find many dead bodies that will likely never be resurrected.

BTW, for many companies, you can often get the code in escrow. So if they discontinue or stop supporting the product, you get the source code.