Back in the day.... decades and decades ago. The hot manufacturing plants would often shut down in the hottest part of summer for a mandated "vacation". This is also when the lines would be "re-tooled" with the new designs and modifications. So the returning workers would then make the "new" models.

I think bicycle models.... are more like clothing fashions... and less like large scale manufacturing logistics. Model changes can reflect technology advances... but are more orientated towards increased (prolonged) sales.