Why did UAL "offer" the 100 seat in these talks?
1. Most have pointed out the current contract limits for 76 seats are or have been reached.
2. It is highly likely UAL has already made a contractual commitment to purchase the 100 seat airplanes and now needs an agreement to fly them with UAL pilots.
It's not a carrot because for the most part current UAL pilots are flying what they want to any way. The only thing that will change is the opportunity for many to fly Captain who currently are too junior to do so.
Now back in the day the 747-400 was held up as a carrot in giving away the small jets and the MEC did bite. No one in their right mind would say today's talks are comparable to those of the past.
Personally I believe the UAL MEC did the right thing to enter into these negotiations with UAL. As many have already stated why wait for as long as 3 years when improvements are possibly on the table today.