I don't think the bandcamp analogy applies. I may purchase a device with the intent of eventually unlocking the bootloader, but not doing so immediately for whatever reason, such as it still being supported by official updates, or the nth feature not being yet removed. Then the possibility of the option being taken away before you've exercised it remains.

1. It's unclear whether the online check is done when you tap the oem unlocking option, or it's done asynchronously in the background. If it's the latter, then it doesn't really matter because it'll get unlocked anyways.

2. There's no real disadvantage to leaving your bootloader in "locked (unlockable)"[1] state, because you can continue using stock rom and avb is enforced, but you can unlock it at any point using fastboot.

3. The Bandcamp analogy still applies because it offers streaming too (ie. web player), which means you could be happily streaming (ie. not downloading) and then one day they shut down, denying you access to all your music.

[1] https://i.ytimg.com/vi/rh6JxG0xE2k/maxresdefault.jpg