Lightning can in fact reduce the number of transactions on the main network, but that is actually a danger, because less transactions, means more cost per transaction to pay miners.
This problem will start before 2140, when the block reward is low enough.
When that time comes they will not be called miners but will have eventually other designation.
Today, each block has an average of around 4000 transactions. If today reward of 3.12 Bitcoin would go to zero, that would mean that each transaction fee, to compensate, would be 80000 sats, around $75!
Assuming that in 2140, all the world wealth will be transfered to Bitcoin, and assuming that each Bitcoin would be valued in todays dollars at $23M, that means that each transaction of 80000 sats would be worth around $18k!!!!
