When will the 2021s be gone?

There is precedent for that happening. However, it was due to union labor strikes, and not because the previous year vehicle was selling so well GM didn't want to stop.

Because of labor problems, GM ran production of the 1969 Corvette long past the planned MY changeover in late summer to December, 1969. This made the 70MY Corvette a short production year, running from January-August, 1970. Total production for the 70MY Corvettes was 17,316, compared to 38,762 for 69MY.

No chance that will happen in this case. Toyota has a few temporary union employees for some jobs, but the plant is non-union. The changeover will be made on schedule as planned. The changeover to the new model will be expected to increase sales even further; that's why OEMs make model changes.

 

My ‘21 was built in November