I’d like to take a moment to examine what would happen long term if we scaled so much that more transaction data ended up in blocks than was produced. In other words, if the mempool shrunk so that it eventually became empty.

Currently, with subsidies from the coinbase transaction, miners are fine mining empty blocks — they get 12.5 BTC (over $50k at the moment) just for mining — but with the halving every 4 years, this will not be the case forever.

Around 2032, after 4 halvings, the subsidy drops below 1.0 to 0.78125 BTC.

2044 (3 more), it drops below 0.1 (to 0.09765625 BTC).

2060 (4 more), it drops below 0.01 (to 0.00610351 BTC)

You probably get the picture. Even with the price of Bitcoin rising the way it is, there will eventually come a turning point where the subsidy alone is not enough to motivate the miners. The intention is, and always has been, for miners to profit from only the fees, after this point.

If there are no transactions in the mempool after a block has been found, the miners will stop mining. There is no reason for them to mine an empty block. If other miners get a head-start mining the empty block, those miners get nothing for their trouble. Nothing gained, nothing lost. If there are multiple chains with the same proof of work (like BCash), the miners will most definitely switch to one of the alternatives until they consider the fees sufficiently high to even begin mining.

The result is a chain that intermittently pauses. For a user waiting for 6 confirmations, this is a pretty big issue. For the difficulty retargeting, it will mean that the chain difficulty goes up and down in 2-week chunks based on how profitable the fees were the last week, rather than pure hash power.

“The mempool will never be empty though,” you say. And you’re right, it won’t. Not often, anyway. But with a few lucky blocks in succession, it gets pretty damn close to it. With scaling solutions, this effect will be amplified. And even if it’s not empty, there will eventually be a threshold for aggregated fees that miners simply won’t find profitable enough to mine. If the reward is much lower than the cost, it makes very little sense to mine, and miners will only become more efficient and sophisticated.

Scaling is obviously the thing to do, but we need to remember that the mempool is a market, not a queue.