this makes no sense. it is like saying in order for a gambling site to start we first need 25% of the whole world to start using bitcoin.
i get what TheQuin says, and it is a complicated thing to implement and extra precautions must be taken before the new feature is added but we already have exchanges like bifinex that have been running their LN node and accepting deposits and withdrawals for a long time. by the way the capacity doesn't have to be huge, it can be limited and also if there are concerns they could only allow the old users to use it not the new ones.