elons vision is not to own a car company or a solar power company

if you think of his ventures as just R&D of a bigger project. and then piece it all together it start to make sense

he didnt start the 'boring' company to make transport tunnels for american cars.
all of this stuff was all R&D spend with ROI to learn how to make tech ready for space colonisation

think about it .. solar panels(colar city), drilling(boring), moon buggies(tesla)
its all in preparedness of funding moon colonisation and asteroid mining for rare minerals.

thats his retirement plan. get rich from space mining, by being the first/main company doing it in space

the end goal is not tesla of solar city. those are just first steps into the R&D required for space mining. where he gets some returns on his investment while doing the research/design

..
when asteroid 'prospectors' want advice, tools, equipment, transport. they will all buy from tesla as the full service business supporting asteroid mining
..
if your thinking 'trillionaire' you are thinking to small.
hyper inflation already made many people trillionaires in many currencies.
US dollar is going to hyper inflate and so trillionaire is the new billionaire.