Probably that free software never actually dies or even really degrades in the traditional sense. Given decades of time, even small incentives to make things better, like the reputational increase one gets from contributing to good FOSS projects, compound really heavily.
Take the Linux kernel as an example. If you were a kernel hacker, even a minor one, from the 1990s, it's quite likely you could parlay that experience into a good job today doing something similar. Those 50-100 hours decades ago have compounded quite nicely for you. But your contribution didn't decay over the next 30 years - worst case scenario, it stayed exactly as good as it was when you stopped, and best case scenario it's been substantially rewritten and incremented upon.
Take the Linux kernel as an example. If you were a kernel hacker, even a minor one, from the 1990s, it's quite likely you could parlay that experience into a good job today doing something similar. Those 50-100 hours decades ago have compounded quite nicely for you. But your contribution didn't decay over the next 30 years - worst case scenario, it stayed exactly as good as it was when you stopped, and best case scenario it's been substantially rewritten and incremented upon.
That's how I explain it to myself at least.