This might be a little bit too sloppy, but I think the idea of Machado makes sense basically as the extreme end of the genre of move that the Pirates should make. That genre is something like "going for broke between now and 2021." It could include signing Machado, making a trade for somebody like Segura (although, he didn't really go for that much, but that's besides the point), or making a big investment in someone like Grandal.
The reasoning is pretty straightforward to me: even in a world where Keller and Hayes both become star contributors, some kind of retooling or rebuilding will need to happen around that time, and there simply isn't enough talent in the upper minors to quickly assemble a contender with some supplementary players. Right now we have a reasonably deep cast of good MLB supplementary players and a handful of guys who either are decisively more than that or border on being decisively more than that. The only coherent plan is to do whatever it takes to get another impactful player and hope for the best with guys like Bell and Archer.
I also really don't think Machado would come here under any circumstance. Bracketing the obvious that Nutting would never do it, maybe one way to go about it would be to give him the huge AAV along with something like opt-outs after the second and third years. As long as he doesn't have a full NTC or something, it will be possible to move him when the window has closed or other guys become more expensive and decisions have to be made, and he has the possibility to take a short-term payday and still attract a mega-contract in his prime. He's 26 years old. There's almost no downside either way, but it would be the most shocking baseball signing in recent memory. I'd be shocked even to see vague bullshit rumors that we have checked in with him.
The market still does seem kinda weird for both Harper and Machado. Obviously Harper makes no sense here, but a weirdly not-as-competitive market for a 26 year old superstar would certainly be a better weakness to exploit than hoping ground balls can get us 10 more wins.