He won't vote. He will simply occupy space. If SD had a D gov., his family would be hounded by the media to make him resign.
This is not unprecidented. Families have refused to let Senators resign before. A Republican in the 70's and a Democrat in the 40's. I guess they like the salary and benefits.
I just fear that the family is not really important in the decision-making process here.
What is not being discussed is the amount of brain damage here. I think it would be nearly impossible for Tim Johnson to be communicative and lucid any time in the next two years.
He might even be down to Joe Biden's level.
I should note that, with the R in the 70s (a SD Senator, IIRC) the R caucus, before too long, stripped him of all of rank and priviledges and committee assignments.