At what time, for which project, over which period? If we're talking circa 1982 for APL between then and now, the number would be greater. From last week to this moment, less. Yet, small odds are always better than no odds, in terms of a thing happening. It really only takes one to get started.

I appreciate you mentioning escrow agreements. That's a valid point. However, "typically" is not "guaranteed". It looks like MicroAPL, a company so involved with APL that they put it in their company name, no longer sells or develops their APLX product (even though the company appears to still exist). They stopped selling and developing it in 2011. Was it guaranteed that Dyalog would take the project over and host the binaries and documentation in 2016? I don't think so. Good on Dyalog for doing it. The fact that nothing was guaranteed is what makes their actions meritable.

Yet, as far as I can tell, it's only the binaries that are hosted. It appears to receive no further development, no support for new architectures, etc. The thing which is materially different is that the source code is not available, there is no guarantee that it ever will be, and no one, outside of maybe one of two people, have legal authority to use the source. Your point about escrow stands and is valid. It's also fundamentally different than a project having an open license.

PS: sorry, I didn't directly answer your question. You asked for an estimate of a non-company continuing development of one of the current freely licensed projects you listed. For something like LLVM, I think pretty high. A lot of people are involved in it, a lot of people prefer it to GCC, and it has been worked on by many people for many years. But knows. Maybe the companies pull out support and it dies. Maybe not.

That's also a separate question: what's the chance of an open project getting continued support. My lament is that many APLs aren't open to begin with and are not guaranteed the chance to get support.