It seems to me the data tesla are collecting are not unique. How many times logging the same driver driving the same 20 mile commute before you're in diminishing returns?
With deliberately chosen conditions, scenarios, and routes, it seems to me Google could be collecting data that are just as useful although maybe covering much a smaller number of miles.
I can't imagine that the data from carefully selected scenarios would be anywhere near as valuable as the data from random, unpredictable, real world driving.
I don't know where you live, but my daily commute is never the same. They either start/end at different times, or I have to take different routes, or the weather is different. I think it would take a lot longer than 3 years before they start seeing actual diminishing returns on mileage collection.
More than 3 years? I would confidently wager that noticeable diminishing returns would occur after 3 days, and very significant diminishing returns after 3 weeks.
Think about seasons, varying weather, light levels, traffic fluctuation, construction, changing parking patterns, delivery drivers/stopped trucks...the list goes on. It would be very useful to have that occur within a relatively known route, where you can isolate and model what isn't understood already.
