When assessing Musk's success, note that Tesla has singlehandedly pushed the entire auto industry towards electric vehicles. Before Tesla, electric vehicles were funny experimental things for hipsters. Very few people bought electric cars, and when they did it was usually only as a second car that could drive 100 miles around town. Virtually no major manufacturer were planning on changing that.
After Model 3, all major car manufacturers are changing their product development and production lines to make electric vehicles. It's now clear to everybody involved that in a number of years, it will only, or almost only, be electric vehicles.
Even if Tesla were to end up being a company that only sells a few hundred thousands cars per year, their impact will have been several orders of magnitude bigger. At the end of the next decade, tens of millions of electric cars will be produced every year and Tesla is the company that not only showed the way but forced an entire industry to go in that direction.
Now, why on Earth would Musk not be interested in promoting mass transit? Tesla is unlikely to produce 50 million cars yearly anyways, and - so far - electric vehicles are more expensive than ordinary cars and not much more suitable for dense city traffic than ordinary cars (although the cost is going down and although self-driving capabilities may change that a bit). But for many years to come, it's unlikely that the Boring Company or the Hyperloop is going to cannibalize on Tesla's market.