When the app server is stateless, HA for the app server is usually a near trivially easy problem to solve though, and a whole lot of web devs are used to treating the database as a black box that they'll treat as always up so I get the fear and uncertainty of suddenly having the HA setup interfere with "their" domain.

Coming at it from a devops angle, I'm used to being the one to have to deal with the database HA myself, and then having one tier less becomes a lot more appealing, and not really more scary than ensuring any other database setup is resilient.