A good friend of mine also happens to be my accountant. When I talk to him about the open source accounting and bookkeeping software I've been working on, he mentions that he has been using the same bookkeeping software for many years. So long, in fact, that the company which created it is no longer in business, so you can't even go out and buy a copy, not get support for it.
But he still uses it. Its too bad that other people aren't able to try it out, and the effort the program designers put into it was wasted in my opinion. The decay of valuable intellectual property in the form of software is usually referred to as bit rot.
Open source software is inherently averse to bit rot because not only is it open, it is also free. Free as in beer, but also free as in freedom. It is free to be copied and modified so that if the owner of the copyright shuts down or stops working on it, the software is free to live on and see another day.
Many open source projects have passed through many hands during their development. That too is a benefit. New perspectives and ideas keep software fresh and relevant for today's standards.
How much of proprietary-only software will be left behind? I'd venture to guess that all of it will. That's too bad. Hopefully more developers will focus their talents on open source software instead.