There was also a risk, with things like mod_php and the like, that a user could hit your app in the middle of an upgrade. Commonly in those days (for many, not all), the app was tied rather tightly to the webserver. That is no longer the norm, it's more typical to reverse proxy to an app server or something like that.

And sure, staging then copying everything would be faster and safer than deploying straight on top of the old version, but that's still a not-insignificant amount of time when the app is in a transient, partially-upgraded state. And that would only get worse as your app grew.

I suppose to way around that would be to deploy twice and use a load balancer that you could temporarily disable, upgrade one deployment, point the load balancer only to there, upgrade the other one, then re-enable load balancing between them.

Man, that's a lot of work, though.

I'll agree that maybe it's a relatively minor complaint, because there are likely plenty of solutions I just never needed to find, but I really do like avoiding anything that can and should be avoided. I'd rather have a "Please wait while we upgrade" page than risking something going south due to some kind of race condition.