Well the rationale is would you rather have a team that is better in 2020-2024 than a 50 % chance at the 2nd Wildcard spot next year. Even though Donaldson isn't having a good year teams will give up more for him at the deadline then they will in the offseason. Biggest hurdle would be the $ involved.
Even if he turns back to a 5 Win player is Martin, Morales and Pillar going to get any better while another year older? They still need 2 OFers, 2 SP and perhaps a 2nd baseman. There is a lot of work to be done this offseason.