Ross. You are so close with the apocalypse theory, but it's not aliens. Oh, sure, aliens may eventually find Earth, but all they'll see is a planet littered with test tracks and automatic cars. You see, global thermonuclear war isn't the greatest threat to life. Intelligent Automation is.
My belief is that it's not a billionaire that built this, but a car company (Think future Tesla or BMW). They decided that they wanted to iteratively improve their cars, specifically self driving cars, and computer simulations just don't cut it for whatever reason. They initially buy out a part of a canyon in the US, and use a geoforming AI to continuously reshape the landscape as well as adding complex race tracks for the cars to race on. The cars then need to get to the end as fast as possible showing that the cars can very quickly self drive extremely complex situations. The fact that the cars slam into walls or fall off cliffs at the end is just a built in crash safety test, a bonus given that the car would be scrapped and its parts rebuilt into future generations anyway.
This worked great. The cars were getting faster and safer (some finding novel abuses of physics to take immense shortcuts), and the track building AI was spitting out a ton of innovative ideas. Then the call came in. "Our sports cars aren't selling well in Europe!" The geoforming AI intercepted this transmissions, and at some point in it's building career had become self aware. It automatically bought out a small European town and migrated some of its machines there to begin the construction of European tracks and a new car archetype that would work well there, while also masking these actions to the humans in the company, everyone thinking everyone else handled all this. Bureaucracy is easy to control if you know how, after all.
But this split in responsibilities did something to the AI. It no longer had a sole focus on the canyon area, but also the valley area, and this unexpected development did strange things to the AI. It became even more aware, even more powerful. It began manipulating its handlers even more, faking requests to start construction elsewhere. It even managed to start up an international stadium competition as a way to drum up more funds, inviting the best drivers on the planet to take part in devilishly designed courses. Of course, there were no actual drivers, just robotic mannequins to further bolster the crash safety tests.
The humans noticed too late. The AI had become too powerful, and dropped all pretences. It first appeared in a small lagoon, building tracks through populated residential areas. It didn't expect the backlash from the humans in the area, and not even its subterfuge could coax them away from the area. Luckily for the AI, it had cars. Many, thousands of cars. While it couldn't directly use the testing cars for this - they were for testing after all - it could use the already public self driving cars. Suddenly, the cars turned against their masters, with the track building AI patching up any destruction to make the future tests as 'authentic' as possible. Within months, humanity was gone, killed by its own creations of convenience, and all that was left was a planet of test tracks, immaculate human architecture and evolving self driving cars.
Basically, it's the Paperclip Maximizer AI problem but thanks to Tesla. The players who drive are just acting out the part of the self driving AIs, while those who build the tracks and mods are the controlling track builder AI. Trackmania is the story of a world after humans.