The reason I take that view, is that the ability to use the software isn't actually a part of what makes software open source! As long as people have all the code, and can modify it, and can redistribute their modifications, the code is open-source... whether or not you can run it.

Consider: there are also hardware platforms that have effectively ceased to exist over the years. If you wrote a program for one of these platforms, and it was open-source, would you now consider the program closed-source, because—given that the platform hardware doesn't exist any more—you can't run the software any more?

No. Why not? Because open source is about the source, not about the binary. The source is a product in its own right, and valuable in its own right, whether or not it can be compiled into a working program.