Unfortunatly nobody has a crystal sphere, but, looking at Wikipedia, I would say that the expected support span is of ~5 years. As the Series 4 is getting WatchOS 10, I would say that the Series 5 will get WatchOS 11.

But… Apple could always change their minds for the worst (stop at 10) or for the better (going further than 11), only time will tell.

Blocked communities about piracy like this one.