"supported forever" is the most practical point, and servers are all about practicality imo. Look at it this way: you'll need to buy a new Mac, or start using Linux, when Apple cancel support for your hardware. Start today instead.
you can upgrade to a newer version, but even that is not 'forever'.
chack out 'rolling' distributions, it's a concept that suits servers very well (OS/software support forever). obviously hardware is not often supported in perpetuity, but if that's the issue, Apple is the last platform to consider
If he does end up choosing a Mac Mini, that's an expensive, but not terrible, relatively low-power, but more capable platform than e.g. a Raspberry Pi, so he would have more computing headroom for other stuff.
One more thing regarding longevity: To the best of my knowledge, Apple hardware gets major version updates for many many years. Even when that stops, similarly to Android, they still ship security updates. For instance,
macOS 11 is still supported, with its first release in June 2020.
no. although Apple provide some long-term hardware support, they have the worst record in the industry when it comes to supporting old hardware (and that's a significant part of their business model).
"Many years" might be enough in practice, maybe we can agree on that
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!