But most of the complaints about that are coming from existing Hacker News users, for whom the experience on mobile is terrible.

If Hacker News was as adamant about stopping the Eternal September effect as some people believe, they would demand references and a CV before letting anyone join, or else be invite-only like lobste.rs. Not having a mobile stylesheet doesn't really seem like it would help in that regard.

Cynicism and incivility are far more corrosive to this community than the Eternal September effect could be, anyway.