I don't think there are any players on the 25-man that you keep if the team actually does go into a down cycle. Maybe Travis if he gets healthy.
Pillar - defense only guy in his age-28 season
Osuna - closer, who is at the apex of his value now. No don't keep relievers through a rebuild.
Sanchez - pitcher, only three years of team control after this season
Stroman - pitcher, only three years of team control after this season
If you're at the point where you're trading Donaldson, the team simply isn't going to be doing much winning. At that point, keeping pitchers that will be on free agent deals by the time the team is ready is bad business. Scorched earth rebuild would be the best option. I don't want the next version of this team to be built around Stroman and Sanchez on 120M contracts.