So if using a static nav mesh then you would be right but as long as you set your recast nav mesh to dynamic, then what you can do is update its scale as your world grows.
So each time your map grows then you could check to see if it is larger than the bounds of the current navigational area and update if neceesary. for the change to be registered you will need to call navigation bounds updated in blueprint to see it reflected.
This in theory could give you an unlimited dynamic nav mesh, however…
This will get expensive before too long so just keep that in mind if this is the route you’re going to take.