I work for a software company delivering several systems which have in some guise been on the market for 15-20 years.
We still have several of the original programmers of the package on staff, and we're only about 50 people strong.
In fact, of those 50 about a third have been with the company for 10 years or more, so for most if not all of the life of the current products.

I think the same is true in larger corporations as well (like Microsoft and Oracle).
For example, Steve Balmer and Bill Gates started Microsoft from scratch and wrote the first products in their own homes. They still work there (though I doubt they still have time to do much coding).
While younger employees are more likely to switch jobs on a whim (in fact, a few years ago it was sometimes frowned upon by career planners to stay with a company for more than 2 years if you're under 35) older employees tend to stick around.

Another example:
I worked for a major bank where also a good part of the team had been working on the same team (this was the development team for the entire IT infrastructure) since its original start over a decade earlier.

In OS projects there is probably about the same turnover in staff.
While the founders are likely to stick to the end, the younger members come and go as their interests shift.
The only difference is that they typically do not get paid (or if they get paid it's by an outside COMPANY, the kind you seem to despise so much) who pay them to contribute to that project only so long as it makes commercial sense for that company to support that project.
Many of the largest and most successful OS projects would in fact flounder rather quickly and messily if the companies supporting them were to withdraw their staff and financial resources.
The maintainers would have to leave to do other work that gets them money to pay the rent leaving the project leaderless with only a gaggle of constantly changing initiates trying to keep it going but failing because of lack of oversight.