Countless companies and industries enjoy making up scary stories when it comes to justifying their opposition to making it easier to repair your own tech. Apple claims that empowering consumers and bolstering independent repair shops will turn states into “hacker meccas.” The car industry insists that making it easier and cheaper to repair modern cars will be a boon to sexual predators.
That assumes that the manufacturers want to bring down prices. If they all keep using proprietary batteries they can use that as an excuse to keep prices high without looking greedy.
Sure, if they want the wraith of the EU on them.
The EU has a tendency to intervene when a market is unable to regulate itself.
This is a purchased item by bike makers. If the prices for the batteries goes down, it saves them money as well. This is why industry tends to jump onto standards because it is better for everyone (USB, NACS, various ANSI, ISO and IEEE standards). Quality (and safety) can go up, prices can go down, and availability of parts increases.