Why do you think the play Death of a Salesman would be something you wouldn't want to read again or something you won't teach to your students?
Death of a Salesman is a thematically rich and very well-written American drama. If there is a reason not to read this play multiple times, maybe that reason would be attributed to a preference for redemptive works.
There is very little redemption offered in Death of a Salesman, much like Oedipus Rex - realization, here, comes just before (self-inflicted) punishment.
Does this open the play up to judgments against its value as literature? No, it doesn’t. Does this mean that some people will be depressed by the pathos and failure of the protagonist in Death of a Salesman? Yes, it does.
Great literature is not always pleasant, but will always move the reader to an emotional response. If a play has disturbing themes and manages to be disturbing then that play is effective, even if it is unpleasant.