Now they only need to make sure that a supply chain for replacement batteries exists, there is regulation and competition and options remain available for a reasonable price.

There are plenty of old Dell and HP laptops with replaceable batteries which can only be found on eBay or some random seller that does who knows what under the refurbishing process.

Exactly. I had phones and laptops with replaceable batteries in the past. I liked the idea of it, but in practice there was no OEM-quality replacement available by the time I wanted one. The device would have been usable to me still, but not with a random black market battery that may well be a fire hazard.

Having thought about this long term, I think the only solution to this would be mandating standardized battery cells. Rather than every phone model having a bespoke cell that is manufactured once and then obsoleted, they need to have standardized shape and electrical characteristics so that batteries being produced for new phones would also be useful to rehabilitate old phones.