So this move seems to be designed to do the following
-Make people book the flights (times) they actually need, so that Delta can control prices more on the higher demand flights. No more booking Thursday or Friday morning Flights and taking later more expensive ones.
-Encouraging (screwing) people to book more expensive tickets because they will be the only ones able to SDC due to fare class restrictions.
-Encouraging (screwing) people into booking High level awards
So can the traveler fight back? Its going to be a lot harder, but if (highly unlikely) there is space in your fare class with a departure within +/-3 hours as a GM+ you can SDC to it for free. Well does this new flight now become your departure time? So Can you SDC from that with in +/-3 and then keep doing so till you get the flight you actually want without ever having to pay the greedy $50 fee? This would be hard to do as there would need to be space on multiple flights for it to work but just a thought.
As for me im really sad this happened but glad to be unaffected, i wasnt going to be lead to the slaughter, i ditched DL at the end of last year after over a decade of loyalty to NWA/DL including charter DM.