> If I correctly get your point, the whole UL/UR system will need to be revised to adapt to more consumers directly buying replacement parts, as they'll expand beyond the nerds following iFixit tutorials.
No, the underwriting and test labs system will not be "updated". User serviceable parts/devices would need significant structural changes to pass testing.
The whole point of testing labs is to give a liability sign-off for manufacturers. A UL listing just says "this particular product built to X specifications did not immediately explode, catch fire, or kill anyone during our testing". It means that design isn't immediately dangerous under normal use co dictions. The UR listing is really just a claim that the component can sit in its packaging in a warehouse (in normal conditions) and not explode, catch fire, and is unlikely to immediately kill someone before it's sent to be assembled into a UL listed device.
If a manufacturer is selling to consumers no retailer, wholeseller, warehouse, or shipping company will touch electronics without some sort of test lab underwriting. In order for batteries to get that underwriting they need to survive physical abuse tests that will cause most LiPoly batteries to explode or catch fire. Look up LiPoly battery fires on YouTube. It's common to use naked batteries with no protective cases in hobby projects.
In order to make LiPoly batteries that will pass those underwriting tests they will need outer impact and puncture resistant casings. For any given envelope for a battery the protective casing will eat into the battery's capacity. Depending on the material and envelope the casing could be up to 50% of the battery's volume.
Internal layouts for some phones would be untenable if the batteries were user serviceable and required a protective shell. The L-shaped batteries in phones like the iPhone XS would be an impractical shape.
The difference between a high capacity battery and bomb is really only the discharge speed. Lithium is incredibly reactive with oxygen. The only reason they're relatively safe to keep in your pocket is they're wrapped in a sturdy aluminum shell.