We're stuck with him, like it or not. He's being paid the same regardless of his production. Nothing as radical as DFAing him is likely. So far he's not injured, so can't just be forgotten.
He's not been used much as a pinch hitter or platoon player and doesn't have the skills anymore to be a defensive replacement.
So it's the starting lineup (DH or third base alternatives) or the bench.
As a bench player he would get minimal at bats and playing time. Maybe platoon situationally. And with reduction in at bats it will be harder for him to find his form. Not to mention probably hurt Rendon's ego and confidence.
It seems Washington has few options. Keeping him in the lineup regularly with some days off may be the most realistic way forward. And dropped towards the bottom of the batting order. And hope he finally gets in sync. But if he hurts the team indefinitely then Washington will have heavy pressure to put his best lineup on the field, regardless of hurt feelings.
Maybe Rendon also needs to work more intensively with the coaches. Perhaps they can correct some flaws, though that will take time.
Home fans probably should at least go through the motions of showing support because it would help the team more than stoking the fires of dissension.