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.<p>It misses one important thing: network difficulty.<p>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.<p>We just saw this happen when China turned off mining.