Well, there are potential issues with people repairing their own phones. They could use the wrong (or counterfeit) part and cause the phone to emit radiation that interferes with nearby communications. Or they could use the wrong (or counterfeit) battery or even damage the battery during replacement, potentially causing a fire hazard.

The purpose is not for you to fix your phone motherboard, a competent repair person with access to tools, schematics and repair software would do that. If bad quality of batteries are on the market the solution for fixing that is not to prevent people replacing the phone batteries, it would be like if bad petrol is sold somewhere and then car manufacturers use it as an excuse to force you to buy "genuime petrol" so your engine would continue to work.

Also if we want to prevent fires because of batteries then we should make Apple products illegal because it happened that Apple products had issues too https://support.apple.com/15-inch-macbook-pro-battery-recall

Typically people aren't replacing the antennas or radio amps, so I'd keep the radiation issue in the theoretical bin.

As for bad parts and bad repair jobs, sure. I would think that owners are incentivized to buy parts that are safe and to install them correctly. Of course some battery batches will be bad, but that's true for authentic parts too. Remember the Samsung Note battery issues?