I just started working part time at my LBS because they were getting behind building new bikes to sell. I am working on 2021 models. Not sure what's going to happen when they sell through those. The supply has been filling back up, for example a batch of new cassettes came in for the first time this year. But there are still things like 26" tires that are in short supply and are being acquired through barter. In general, the more expensive a part is, the easier it is to get. That tells me the demand side is important to understanding the shortages.

I don't really understand how bike manufacturing works, even though I used to work for a bike manufacturer. But the increased demand from new or lapsed customers in combination with increased demand from existing customers makes for shortages. There were big shortages back in the early '70s bike boom, this is similar to that. A couple of years later it had died down and the suppliers had caught up.