This is interesting paper. They already anticipate spammer use specialized hardware and buy only needed computer parts (no monitor, small HDD, etc.) for sole purpose of solving PoW. Although the most interesting part it'll bring inconvenience to 1%-13% legitimate user.
On a side note, i wonder how DAG-based cryptocurrency (such as IOTA and Nano) handle specialized spam attack since user need to verify 2 transaction rather than pay fee when creating a transaction.
Both Nano [1] and IOTA [2] use PoW for spam protection. Or at least they used to, not sure what the current state is or how it accounts for improvements of hardware over time. Beyond that I guess having a central coordinator helps.
[1]
https://blog.nano.org/nano-pow-the-essentials-6bf8b021d49d[1]
https://legacy.docs.iota.works/docs/getting-started/0.1/transactions/proof-of-work