That's it. But - the largest part of the reason I (and, presumably, some others out there) own Macs is that they run Apple's OSes. If I want a Linux box, I can use any old x86 hardware I happen to come across and get a much better experience than on a PPC Mac.
I feel like this is another great divide among not only this forum, but for the rest of whoever else continues to insist on using their PowerPC Macs into the next decade.
We have different reasons for using the same machines, all just as valid as the other. Evidently, this also means that they have multiple qualities that appeal to multiple types of people with different priorities. You've hit the nail on the head in that you want to run an old Apple Mac to run old Apple OSes. It simply comes down to that for you and many others, because that's the central priority. And there's nothing wrong with that at all.
I (and I think a smaller amount of people than the former group, including Dr. Kaiser), can live without Apple's old OSes and only see them as a plus to the differing central priority, which is to - regardless if they're Macs or not - drive well-built computer machines with badass RISC processors that had roots in the server markets, high-end workstation areas, supercomputing sectors, and even space exploration.
Case in point, if I came into possession of an IBM IntelliStation POWER machine, or anything by Raptor, I would drive that instead because it more directly appeals to my tastes, skipping the Apple branding, apps, and OSes in the process, leaving just pure POWER and UNIX heaven.
And that probably mostly explains my work with Linux / BSD on PowerPC, because the machine itself is the goal. Not the OS.
A modern AMD machine isn't vulnerable to Meltdown, will get fixes and can be had for pretty cheap and will run basically any linux distro you want up to date easily. Relying on a 15 year old processor for security is absurd.
You miss the point of "preference".