ADA requirements trump most other aspects of design now. Unless your cars are grandfathered in and as the years go by, fewer and fewer of them will be.
And, yes, common sense would tend to indicate that decreasing weight by 9 tons would make meeting the buff test standard very difficult without spending more money on advanced design or more expensive materials. But if the acquisition board doesn't know what they are doing they can demand anything they want and then blame the supplier when the supplier tries their best and falls short. If each car weighs 9 tons less then the acceleration figures would have made a noticeable change in scheduled times for routes with a lot of stops, but lighter generally means less robust or more expensive.
Flip side of the coin, Nippon Sharyo knew the bid was going to be very tough to deliver and made it anyway.
Isn't ADA accessability to the lower level seating and restrooms sufficient in the present cars? And it seems unrealistic to expect to lose that much weight and not sacrifice strength....