Let the car be developed by a centralized automotive industry, but give the backyard mechanic the option to tweak some parameters that are developed by these specialists. (They can play around with that, based on your driving style)
Most backyard mechanics cannot work on new cars today, because it is all electronic these days and everything is done by the onboard computer. (These guys do not have the tools and software to work on these cars)
To offer a different perspective. Years ago, it was claimed open source code contains fewer bugs and security vulnerabilities per lines of code. In contrast to proprietary closed source code. Open source mozilla firefox was claimed to be more stable and secure than proprietary microsoft explorer. These claims were partly attributable to greater peer review and scrutiny of open source code.
Linux(open source) versus windows(proprietary) is another facet of the debate. Bitcoin itself is considered open source software, if I'm remembering correctly.
I suppose the worst that could happen if tesla ever open sourced its software. Is if Roger Ver forked the project and named it tesla cash. Tesla software specially designed to buy coffee at starbucks.
A good chunk of open source relies on volunteer based man hours. Its more like volunteers developing counter strike as a mod. Than simply hiring more workers.
If tesla software were open sourced. Volunteers would develop mods and hacks for the software. Which would be scrutinized and tested before ever being released to the general public. Its a win/win scenario for everyone. Volunteers gain experience and credibility for their contributions. Tesla consumers get free stuff. Done correctly everyone benefits.