I completely agree that open sourcing the project does not guarantee that the project will continue to function. I also think that after a company goes out of business they have no responsibility to keep their project functioning. Once they open source it, anyone is free to put in the work to keep it going and I can't think of a better guarantee a company could reasonably give.
How many application variables are hard-coded to your company's specific network and server configuration?
Have you fully documented the configuration of every related server, checked that in, and kept it up to date?
Are the required versions of every dependency well documented?
Does it use any third-party packages that you have modified, but never checked into your source control?
Is any part of it dependent on some massively expensive piece of software or data set that you happen to have easy access to for some reason?