Have 6 releases broght exponential improvements?.
Lets see:
Same "full day battery".
A few millimeters larger screen
Always-on screen
Blood pressure sensors?. Nope.
Blood sugar sensors?. Nope.
Huge performance improvements in user experience?. Nope.
New ways to interact with the watch?. Yes, sensing the wrist to identify a "virtual" click.
Following Apple working way, what do you think will bring Watch 16?. 1.5mm larger screen?, real 1 day battery?. If marginal improvements keep comming, a Watch 10 might easily last 6 years without needing to "upgrade".