Incompatibility of Apple Watch Series 4 bands with Series 11

It is time to upgrade my series 4 watch (40mm) but in true apple fashion I am unable to use the numerous expensive watch bands that I have. Why do they do this? New series comes out, the old starts acting up but the new is not compatible with bands that you have spent hundreds of dollars on. Will there be a way to make the series 11 (42mm) adaptable?

[Re-Titled by Moderator]

Original Title: Series 11 Watch