He just turned 24, doubt he quits because of one bad year.
His year actually may be worse than Golubev's (although he beat him this year). 9-31 on the year is just crazy considering that in 2006 he had a 73-31 match record.
It's clear to me he hates tennis, and he has no game left anymore, he's not like Golubev who can suddenly find confidence and form.
Zverev has a bad game, even his best now isn't top 100 level, and mentally he's done.
Unless he suddenly changes his mentality he SHOULD quit tennis completely.