The fact is that Tesla simply isn't prioritizing user replaceable car parts yet. That's fine, as Tesla has a grossly different business model than Detroit.
Replaceable parts is the cornerstone of long-term reliability of machines. And it takes a lot of technology, design, and engineering to make that happen. Electric cars, as a newer technology, don't really know what parts fail in the long term, or what should be or shouldn't be replaceable yet.
In any case, I can imagine a set of gears that doesn't have any supply chain, and may be hard to replace. Just because a transmission is made of mechanical parts doesn't mean its actually practically replaceable: you need suppliers who are building the parts as well as mechanics (or books / guides) that share the knowledge of replacing those parts.
Similarly, I imagine that future electric cars will figure out what should be replaceable. Tesla clearly hasn't figured it out yet however.
But that being said, a lot of Tesla customers are wealthy and they are probably happy to either get a new one, or send back to the factory/certified repair shop.