If employees shouldn't complain about not receiving benefits they didn't negotiate for, surely they should get to complain about not receiving benefits they did negotiate for. Knowing anything about benefits administration is not the employee's job and it being "complicated" is not remotely their problem. If the terms of MailChimp's acquisition were incompatible with the terms of its employment agreements, then it shouldn't have agreed to the acquisition. Or it could make an honest attempt at recompense such as through bonuses. But they don't get to both eat and have their cake.
I agree with that. But employment is generally at-will in tech. Not fixed-term. Would he prefer to be terminated? When you go from one company to another there's a change in benefits. He could have COBRA for a period of time and then be back on the payroll of the acquiring company. That's no better.