i wouldn't even wear a smart watch if it didn't give me notifications for iMessage, HomeKit, and other notifications. You use yours for fitness, so naturally a dedicated fitness watch makes sense because it doesn't use battery for stuff you find unnecessary. An Apple Watch is far more than a fitness watch, which is why it can't do 30 days of battery. It is online constantly moving data. i doubt the Garmin bothers to wake up its radio until it's time to transfer data to a phone. They're apples and oranges. To me, the Garmin is useless because it doesn't do the smart watch features that I consider mandatory.
A more apt comparison would be a general purpose tablet like an iPad compared with a basic eInk Kindle. The Kindle is great for one thing, but one thing only while an iPad can run the Kindle app, but also a million other apps. The Kindle's battery lasts forever while the iPad has to be charged daily. It's the same principle. You buy what is important to you.