"Many years" might be enough in practice, maybe we can agree on that
Agreed.

And lastly, even when that support ends, technically Bitcoin Core will still run on it. Bitcoin Core doesn't really care whether your OS is still updated or not.
sure but some hacker will care about your OS having known security holes: but for the wrong reasons!
Again, though: if you view the Mac Mini as a compact, relatively low-power PC with quite a bit more power than a Raspberry Pi, that can also runs Linux, it may be a good choice for some. Because you can definitely install latest Linux kernel on old Mac Minis that don't get software support by Apple anymore.
Even the M1 Mac Mini already runs Linux thanks to
Asahi Linux.