Are the websites about something that may go away? I mean, is it about recipes for the most recent culinary trend? Or strategies about whatever is this winter's trending shooter game? Or programming on a language that may start to die in 5-10 years? Or a framework that everybody will drop in 6 weeks?

It's hard to predict something that generates X income from visitors today will keep generating >= X income in the future.

Is your revenue mainly from Google or something like that? I have heard quite a few stories about people suddenly having to change ad networks and then having their income drop. Or Google changing something in their SEO algorithms and then the site dropping. I even remember reading something about Facebook making a change that stopped most of the organic visits to a specific site. Are you subject to that?

If any of the cases above might be true, I would pretty much only quit my job if I had "can retire in a few years" income.