meaning engineers know exactly what amount of fuel would be needed and when, per flight.

There is no need for fuel depot in space, as long as this philosophy keeps going.

A fuel depot implies there are other possible missions for spacecrafts that have been already used.

The Shuttle is re-usable, but we very quickly understood that re-usability isn't the panacea! It means repairing, long hangar occupation...

A re-usable tug implies it must return back to the fuel depot. No spacecraft so far, has ever returned back to Low Earth Orbit, from GEO or from the Moon or from a solar orbit. It means it's going to have to slow down in such a way it can rendez-vous with the depot, while coming back from far away!

GEO satellites can make money without a space depot.