The entire planet's worth reverse geocoding data is ~120gb. The map tiles file for whole planet is also ~120gb, and they both are precompiled, so you don't need hundreds of gbs of RAM to run your local planet. It's easier than you probably think nowadays. Not mobile-size, but local server-size
I'd love it if it were easier than I think, because I spend a lot of time thinking about it! I host maps.earth, which is a planet sized deployment of Headway mapping stack (which I also maintain).
To first order, you're right on about the storage size of a vector tileset and an geocoding dataset based on OpenStreetMap. But Google maps is a lot more than that!
Headway uses Valhalla for most routing. A planet wide valhalla graph is about ~100gb of storage. It doesn't produce reasonable transit directions. Transit is an even tougher cookie.
OpenTripPlanner gives good transit routing, but it doesn't scale to planet-wide coverage. We've settled on a cluster of OTP nodes for select metro areas - each one being on the order of 5-10GB of RAM.
So, I'd say we have some of the pieces of a general purposes mapping tool that could replace Google Maps usage, which you could host yourself.
But we don't have satellite imagery, real time traffic data, global transit coverage, rich POI data (like accurate opening hours, photographs, reviews).
Do all people want all these features? Probably not, but a lot of people seem to want at least some of it and it's not obvious to me that they'll be quickly solved.
