Well in that case, you can get a traditional sprinkler controller, and you can run OpenSprinkler with another controller in parallel. So if either controller turns a zone on, it can run that zone.
There are some subtle issues regarding the electrical part of running two controllers in parallel, but in general, as long as each of them uses their own power adapter (i.e. don’t share the same adapter), it should be fine.
If you look around for web-based sprinkler controllers, a lot of them don’t even have a screen nor any buttons. So it’s not like OpenSprinkler is any worse than them. If anything, it at least has a LCD screen and three buttons, and you can do some basic control using the buttons.