Two suggestions for you: 1, VMs are your friend. 2, decompilation technology has gotten very good. I'd "statically analyse" the binary in a disassembler/decompiler for a while first and figure out what it's doing before actually trying to run it.
I don't think source is necessarily always making things easier either --- I've had a few times where, even with open-source software, it's easier to find the right bytes in the binary to patch than to figure out where in the (huge) source that would be, and then how to build the rest of it (along with all its hairy web of dependencies) completely unmodified from the original binary.
Especially if it's a fundamentally trivial change (like a string constant somewhere, whose desired value is the same or smaller in size), and I don't expect to make any more complex changes, I'll definitely choose opening the hex editor for a few minutes (at most) over spending perhaps hours downloading a few hundred MB of source and dependencies and figuring out how the original was built and how to reproduce that.
It technically isn't, much like having an excavator isn't essential to digging up ground for a new mall, when you can technically do this with a shovel too.
I get your point, but right now, I'm sitting in front of a 22-years-old game I spent a great deal of time even trying to get to run. I want to restore it. There's no source leak of it that I know of, it has no clear relation to prior titles and there wasn't any reversing effort I could find either. Half of the formats used by it are obscure, and from causal inspection seem to be dumps of in-memory structures.
Having source code, my main problem would be to build it - but it's essentially a straightforward task of finding and patching or mocking various 1990s-era peculiarities, until the whole thing builds correctly with an reasonably current compiler. The source code would assist me with reversing the data formats too. However, I don't have the source code, just a barely-32bit application with a 16bit installer. The game plays really weird tricks with your screen, so attaching a debugger will be a PITA (unless I figure out how I can run a debugger on a different machine and remotely debug the game on the one computer that it manages to run in half-broken fullscreen mode). Best I can do now is poke it and see what changes.
> It could even be argued that the rise of open-source lead to a decrease in interest and skills of RE.
I think so too - simply because being able to ask decreases interest in and skills of figuring stuff out yourself. Has its good and bad sides.