The 5% being people travelling from Brighton. And possibly Lewes.
The 95% being the people travelling from anywhere north of Brighton, or whose trains join the BML north of Preston Park coming from (e.g.) Hove, Haywards Heath, Littlehampton, Bognor, Portsmouth, Southampton, Eastbourne...
If they re-opened Lewes - Uckfield (and electrified it) and opened a chord to allow passage from Angmering to Arundel and the line to Horsham without reversing at Littlehampton then it would help drastically in these situations.
BML2 is a waste of money and doesn't address 95% of the issues...