On the Macro guys fall off a cliff mid 30s. When you look at the micro a guy like Beltre is projecting for a 4 win season next year for his age 39 season. You throw the blanket aging curve out the door and delve into Donaldson and determine what tools makes him as successful as he is. How will those tools look 3, 4 5+ years out. You're relying on your scouts and taking on big risk. It could be an epic flop or it could provide tremendous value. JD could be a 5 to 6 win player 3 or 4 years into his free agency. At that point his contract will provide huge value. You pair that with Vlad, Bo, Alford, Pearson...etc and that's how you build playoff teams. If you're wrong, you could be in a scenario where 3 to 4 years into a JD free agency contract you are trading off the Vlad's to get prospects for your next run.
At some point you have to gamble. I'm not sure if a JD gamble would be the right move or not. I do think given his age, you might get discount. Manny likely gets a higher AAV for 10 years than JD gets on a 5 year deal. He certainly gets a higher AAV if JD gets a 6 year deal. I think JD is easily the better player for the first 2 years. He might be the better player for 4 or 5 years of the deal. If that's the case, you've won.