Actually you're both right. There's a lot that OpenXcom is, can and could be, but I'm gonna avoid sounding incredibly ambitious.
Basically yes, the main goal is to reproduce UFO in its entirety with new clean code. But then what? I've got this shiny new codebase going stale. Why not start adding features and improvements people have always asked for? Why not add support for new resources, graphics, and more? Why not let people mod the game, turning OpenXcom into various possible alternate game modes? Etc.
That sounds like every other remake's goal though. Where OpenXcom differs is that remakes always take their own approach on the UFO formula and that's it. OpenXcom will always,
always, support original unadultered UFO gameplay, with any extras or additions that might interfere with that being completely optional.
If you're familiar with
OpenTTD, that's the sort of thing I'm kinda going for.