Not sure why they had to go in the first place - they had a number of things going for them including:
1. Comfortable seats
2. Smooth ride
3. Decent room
4. Lots of seats - 805 in a 12-car set.
5. Fast acceleration - look at speed from 0:11 in this video:
I also heard from earlier threads that they are similar to 323s which can get 1 min 40 to 90mph on a flat track. In any case this is around the same as a 700/slightly better than a 387.
6. Clear 1st class with a benefit - bigger tables and seats.
The 387 offers air con, and a 110mph top speed but that's it. Why get rid of them? It was a mistake