It's not a matter of hardware, it's of software. The impact of players grows exponentially as you add more, and a better CPU or more RAM can't solve that. The hardware we use is already enough to hold over twelve thousands of players simultaneously, something that even retail servers didn't do, but software performance just deteriorates both over time (hence why there are restarts, and why even your Windows gets slower and more prone to issues the longer you keep it on, no matter your CPU and RAM amount) and with amount of people simultaneously interacting with it. As has been said before, twelve thousands is already pushing it too far and people can already experience issues at times when the server is full. Ideally the cap should be lower, not higher.