I agree with you, I think last year was a catalyst for restaurant workers to make the change they would have made eventually anyway.
Something that's always bothered me about this thread is that it's only about servers. There are all kinds of employers that are having a hard time hiring for entry level jobs. So it's too simplistic to say restaurants treat employees poorly and therefore there's a server shortage.

Other than the $1200/month government stipend, the other variable that changed was that the server job was, for all practical purposes, eliminated by various state governments off and on for over a year. Other businesses were affected too (theaters, event centers, etc.), but none that affected as many jobs as the restaurant industry.

Anecdotally, something I've started to hear around my area (where many vaccinated people are still wearing masks while walking outside alone even though it's not required), is that many parents aren't allowing their teenagers to work because they're still concerned about COVID. These kids likely aren't even collecting the unemployment stipend, so that's not the motivation. But they make up a pretty large percentage of the candidate field for chain and fast food restaurants.