As long as you download it now, then you can keep the game as long as you’re a member of PS+. So if you even think you might play it in the future, download it now. Once it rotates off the subscription, then it won’t be available for download anymore (but people who already downloaded it can keep it).
Wikipedia is another example. One of the most heavily visited websites, operated as a nonprofit. Obviously they don’t only rely on donations from visitors, as they have a whole foundation. But they are a great example IMO.
I haven’t played Myst since the 90s! Such a great game.