I think mathgeek is talking about descent in use not descent in price.

Indeed, [1] shows inherent issues with coins that rely purely on transaction fees for eventual security.

[1] On the Instability of Bitcoin Without the Block Reward http://randomwalker.info/publications/mining_CCS.pdf

And more recently, [2], which curiously only considers abandoning PoW in order to deal with issues caused by dwindling block rewards.

[2] Beyond the doomsday economics of “proof-ofwork” in cryptocurrencies https://www.bis.org/publ/work765.pdf

If the market were perfect the coin should become valueless if 51% attacks are happening?