As the DM I couldn’t be happier.
They basically sold samples of 4 different strains of a gnarly zombie virus, the lab used to make them, and all the notes the crazed wizard used to make them. And 1 of the party members almost died because of one of the diseases on the way back to town because he was out of spell slots, the disease kept lowering his CON stat, and part of the sickness was preventing him from getting the benefits of a long rest.
Your party 3 sessions from now
Their party 4 sessions from now:
Frfr this is a great opportunity for the dm to tpk them and make them start new characters in the post apocalyptic world where they have to survive and take down the crime lord while figuring out how the world went to shit. Paint their old characters as amoral monsters who’s lust for wealth above all else caused the collapse of society.