According to Erik Voorhees, the current speed of the lightning network is two pizzas per second. Because he couldn't pay for more through the lightning channel.
"I have had some conversations with Core developers, and they want to change the 21M limit after BTC experiences two reward halvings, which means in five to ten years."
https://bit.ly/2OjGNbd
It is necessary. The early incentive is diminishing by design and time limited. By restricting on chain scale, they must remove the 21 mil limit, change coin reward to a constant.
