Well that's essentially a philosophical question. Depending on what any one person means by the term "disappear." It's virtually impossible that the 3.22 source will vanish, slightly more possible that the domain and actual (official) hosting will expire and go away, more possible than that that the site will get cracked and a trojan introduced that doesn't get caught downstream for weeks or months, considerably more possible that new email features or exploits arise that 3.22 can't handle, and most possible that age-old bitrot will eventually make it hard or impossible to build with a current compiler/toolchain.
In those last few scenarios, procmail itself doesn't have to disappear in any real sense; rather users will just slowly atrophy and move to competing solutions that don't require jumping through hoops to maintain a robust filtering system. When it's too big of a pain, people would just be using something else. Natural selection.
The procmail users periodically discuss resuming maintainership, but so far no one has. I guess we conclude from that that the pain-point hasn't been reached. But it might still someday.