thinner apple watch = shorter battery life, this time 8 hours or less?
Not necessarily.
Apple can very easily increase battery life by just (finally) giving Watch a big SoC and spec upgrade.
But instead of doing substantial upgrades for Watch every 1-2 years or so, Apple usually resorts to giving Watch minor, almost pointless upgrades and pockets the profits instead.
Android watches are software-blocked from full Apple Watch-level iOS integration and also lack the numerous, high quality sensors that only Watch offers.
Having an very strong, (indirect) monopoly on the smart watch market, Apple can then utilize the vacuum that this lack of upgrades create to then create massive sales in the years that Watch does see genuine, big upgrades, like we saw with Series 4 and 1st Gen. Ultra.
Apple are masters of teasing out upgrades as its far more profitable than doing cutting-edge every year.
And with devices like Watch literally requiring an iPhone, it's not like consumers are free to pick and chose.
I'm not saying we're literally locked in the Walled Garden. But the walls sure are tall and expensive to climb, too expensive for most of us. Apple profits greatly from that.