I think it is a stealth upgrade. Apple added or tweaked the AirPods hardware to add two features:
- Lossless audio transmission that only works with the not-yet-released Apple Vision Pro
- A USB-C Port instead of a Lightning port on the charging case.
Why would they do it stealth? Should they announce the new and improved AirPods 3 with these two major improvements so everyone can know that they should get the latest and greatest? I don't think so. I think they'd be heavily criticized for promoting an "upgrade" that most people don't need and won't/can't use. I think they have a real upgrade in the pipeline that they'll want to save the big announcement for.
Apple does sell replacement cases for AirPods Pro. They just don't sell them as a product in their store. You have to go through service, or buy the replacement from a third party like Amazon. Apple's website says:
You can replace a single AirPod (left or right AirPod) or a Charging Case for a fee. Use our “Get an Estimate” tool to review potential costs if you get service directly from Apple.
I don't see why they wouldn't continue to do this.