Typically you get like 3-6 months notice on a parental leave, so you've got plenty of time to find a contractor/consultant, compared to the standard 2 week notice from the much more likely scenario of your expert quitting for greener pastures. If you have such high risk riding on a single employee, you're probably the one to blame: why aren't _you_ (or an equity partner) the expert? Do you not have anything else whatsoever that could be done in the meantime if core development halted/slowed for 6 months? Do you realize an ffmpeg expert can earn twice as much just about anywhere other than your yet-to-be-profitable startup and gets recruiter spam from big tech companies on linkedin every month? Can you even afford benefits and severance for a full time employee in the first place? etc.
Blaming a hypothetical business failure on a parental leave is really just finding a scapegoat for one's inability to take responsibility for their own failings.
Being in a tech hotspot is irrelevant. I've worked remotely for people in Boulder with a coworker living in Vermont. Again, if you want to run a high tech business from rural Wisconsin with no remote workers, that's on you, and has nothing to do with parental leaves.
I don't know anything about ffmpeg and I am confident I can not only learn it but train someone in it in two months. Bad employers hire people for their current skills, and are caught unprepared when people leave for any reason (including realizing they're a bad employer). Good employers hire people with the expectation of training them and letting them grow into new roles. Bad employers rely on the brains of individuals. Good employers document their processes, enforce knowledge-sharing procedures (code reviews and runbooks and support rotations, in tech), and otherwise proactively mitigate business risk.
Imagine you had that same company and your expert was hit by a bus and instantly killed. Don't run your business with a single point of failure.
How is that different than that person getting hired by Google half way through? Other than the fact that with parental leave you generally get 4-6 month warning that it's coming and with someone change jobs you might not even get 4-6 weeks. Also at least in my experience people on parental leave will answer the phone/email if its important.
Let's be honest, anyone can come up with ridiculous hypothetical situations where "obviously" parental leave is the only evil in the world and must be banished. But at the end of the day, they're just that: hypotheticals with no basis in reality. I've seen businesses succeed and fail and I've seen parental leaves in these businesses. Parental leaves are simply not as deciding of a factor as one might like to fantasize, and anyone who wants to argue against them might want to look at the silver spoons in their own mouths before casting stones at others.