Ever seen "The Fast and the Furious"? I am pretty sure people upgrade their gasoline cars all the time. People already replace modular parts on their cars similar to computers. OEM upgrades that increase acceleration, etc may not be common, but aftermarket upgrades that increase acceleration or horsepower are extremely common.
I'd bet there are many more options for upgrading an old 90s model Toyota Supra or Mitsubishi Eclipse than there are for a brand new Tesla Model S.
It's really pretty standard. I have several friends who "chip-tuned" their diesel engines for significant HP boost, people regularly change suspension parts, exhausts, they add intercoolers to the engines, order a turbo etc. Those are pretty standard things and there are many specialized shops when you can drop your car for a few days to get those done.
You shouldn't trust your opinions on the topic if you haven't heard about it because those are really common things among car enthusiasts.
I believe Tesla's idea behind solar panels is to extend the range a bit (5 percent or so) in order to alleviate the range anxiety drivers have, not to charge the batteries fully. At least not before the batteries and panels get much more efficient.
As for IP, Tesla doesn't need to own all IP from lithium ion batteries to chargers. Just to have the necessary licenses for whichever part of the the base IP which is not already in free use and to have licenses or patents to the way they have put it all together (the way they combine the batteries, their charger etc.). Take Volvo: For them, building a network of chargers covering their markets would mean an investment of billions of euros that they don't have. Buy batteries from Tesla instead, and they can pay as they go.
Changing modules in an electric will eventually be like changing RAM or a fan in a computer. Something most people can't do themselves but that an expert can do in half an hour. Electric cars can be built like PCs where one module can replace another. Gasoline cars are not as modular. Also, the 300 billion dollar market for gasoline car parts is mainly because there are many and many old gasoline cars and because their moving parts need to be repaired and replaced often, not because they are upgraded. You rarely upgrade a gasoline car - you mainly fix broken stuff. I never heard of anyone handing in their gasoline car and picking it up a few days later with faster acceleration and longer range, just like Tesla now offers with their new battery.