I think the fact that Brighton usually has a very good service means rather too many people have just assumed they'd have no trouble getting home, even if they left it until the last train of the night/morning. Even a 12 car 700 can't work miracles!
Having been there today, there are enough posters (and on-train messages, platform screen messages at other stations) to warn people that it is/was going to be extremely busy.
Frankly, if I was going to see a gig that ended around 2300, I'd have been looking at booking a hotel in Brighton or nearby. Or just planning to stay up and get back in the morning. It's not as if there aren't enough places in Brighton to go in the small hours.
Good luck in finding accommodation - unless booked about 6 months in advance. In this weather - staying over on the beach or whatever , could have been an option that was "do-able" , but really in this sort of major planned event (which is tremendous news for the local economy etc) - some hard advance planning on (a) ensuring the main line is block free (b) Absolutely maximising the number of extra services you could run towards London is imperative - whatever it costs - and getting a derogation off the DfT to maybe thin out at some other times to create resources, or (as I have done in the past) pay whatever you have to to get the staff out , ideally with their managers out with them to show support (as I have done ,many times for Wembley , New Years Eve , the death of Diana etc)
Yes it made the headlines (as ever) , yes there were dissapointed and delayed people , but everyone got there and most seemed to have got back. (at some time) - so the railway did something right.
Good luck with trying to tone down the event ! - people want to get out there and enjoy , commune etc