Even if Tesla begins using a cloud server for navigation routing, they would likely still need to provide an offline navigation database in the car - and use that for routing when Tesla's cloud server isn't available. And then when a connection becomes available, the route could be recalculated quickly using the up-to-date cloud data.
We already have this situation today. When entering a navigation destination, if there isn't Internet connectivity, the user interface restricts the destination input to only the information you have local in the car - allowing you to enter an address - but it won't be able to do a search on POI (Points of Interest) names, which requires Internet access for searching.
As for Tesla building their own maps - even with car data, Tesla will still need up-to-date maps to provide all of the information needed for routing. The car data won't be able to provide everything - items like building addresses will likely still need access to up-to-date map data (which Google has).
