They are not bad imho. Except for big players (mostly due to massive bundles/discounts/memes) like Nekopara or Hatoful, VNs are alle in the tens of thousands on steamspy. At every sale Higu chapters improve their amount by 2-3k and all of them are slowly crawling to 20k which is not bad.
A different argument would be Umineko, which indeed is moving slower, but it's not the greatest of the versions and with the second half still pending, 5k is probably within expectations considering sales on their site too.
Things like MYTH have obviously no way to spin them, commercial failures and that's it, not everything can be Morgana (carried by word of mouth alone due to quality). I'd be interested in seeing MG approach on Kickstarter and how other more recent 07th titles perform, hopefully with better treatment than just a translation and bad sprites...