This is a well written article that paints a uncertain long term security picture for PoW coins with capped supply. While bitcoin may or may not maintain security thanks to high fees resulting from frequent congestion, the picture is much bleaker for capped supply altcoins, that generally command negligible fees (just to deter spam).

This article actually isn't bad.

It misses one important thing: network difficulty.

Even if it is not profitable for miners to mine due to lower emissions, the difficulty will adjust and the relative security of the network will maintain itself.

We just saw this happen when China turned off mining.

Network difficulty adjustment is there to ensure we get a new block mined, on average, every 10 minutes. It does not impact the cost of a 51% attack, just the block mining rate.

As emissions drop, less money is spent on mining and a 51% attack becomes cheaper.

When China turned off mining, mining temporarily became more profitable as it took some time for miner spend to get back to a equilibrium state (where miners, in aggregate, spend nearly the entire block reward on mining costs).

It did temporarily get 'cheaper' to conduct a 51% attack (although it was still so expensive as to not be viable - due to the currently high block reward). This wasn't because of the difficulty adjustment though - that just maintained the average time to mine a block at 10 minutes.

No, the article is not ignoring difficulty. It simply assumes that the money spent by miners, and hence the network hashrate, will faithfully track the block rewards.

The relative security is NOT maintained because while the hashrate declines over coming decades, the profit from an attack will not as long as bitcoin maintains its price.