We are already hearing about the Apple Watch 6. AW5 just came out a few months ago (Oct 2019) and I had purchased AW4 in January of that same year.
If you like paying Apple money for trivial annual updates, then maybe keep buying. Not sure if we can stay on the Apple Watch Bandwagon. More on the latest version here
SUGGESTION- Buy a nice Swiss or Japanese watch that keeps it’s value when you are young, when you are old it will have appreciated in value and sell it!