Exactly! Otherwise it's a 'how long is a piece of string?' sort of question. I have a 6 core w/64gb of RAM and it works very well for me. I don't do massive live VI templates with multiple libraries w/ multiple mic setups etc. I'm on LPX and typically run a combination of, at most, 50-70 audio tracks, along with maybe 15-20 live VIs, and lots of plugs. It handles all that reasonably well, so I'm good. I will worry about what's next in 3-5 years.Depends on the price its still a great computer.
If you want to do massive live (VI) templates without a slave, get a new Mac I guess. Otherwise, I think a refurb vader for around USD$2500 is totally worth it. I can't justify spending $10k+ for an audio computer (video is a different story). But that's me.