> Do we really see an important amount of stolen batteries in lawfully owned and maintained phones to the point a DRM on batteries would benefit legit users?

The stolen batteries have to end up somewhere and official spare part supplies are really expensive (unless you are a certified partner shop of a specific manufacturer of course), so it's most likely they end up distributed into the grey market.

If I were to decide upon a global regulation, I'd say that spare parts have to be made available under FRAND terms (so, no more preferential pricing) and all valuable components have to be reasonably e-marked, that there be a public global registry between device identifiers and associated component identifiers, and that when someone presents a proof-of-purchase plus a police report, all components get denylisted... and when a device recognizes a change in its parts, the component's identifier is checked against the global denylist. If there is a match, the device's user gets a warning, and the owner of the original device who made the theft entry gets a notification, let police do their job then.