I could see this being a viable substitute for your Garmin.
Yes and no.
I'm an Apple guy through and through. I've owned one or more Apple products continuously since about 1987, including through the early and mid-90's when it wasn't cool. However, the Apple Watch is not something I have any interest in.
The AW is an awesome smart watch that kinda does fitness ok. We are not the target audience. Unless and until it's got significantly better battery watch both in regular mode and in GPS mode, I think it's a non-starter for folks like us. Don't get me wrong. For people like my wife who runs 3-4 times per week for 3-5 miles, walks the dog and does stuff like boot camp, it's perfect for them. She loves hers. But, for people like me, it just doesn't work. I'll take my 935 over an AW any day of the week - or maybe two weeks, i.e. the 935's battery life in watch mode. I prefer a top shelf fitness watch that sort of does basic smart watch ok.
On seekingalpha (investor website), they are talking about $11B revenue from this watch at $350 Average Selling Price (ASP). That sounds like over 30M units that they plan to sell, so to get to that number, they are not targeting the 30M tri geeks and extreme endurance people that don't exist (maybe there are 1-2M of us across sports).
Personally I really can't see the utility of cellular connectivity when I run. My runs are short enough that I am back home/office before any major emergency can get out of hand and I have my phone with me in my jersey for bike rides in case I need to call. I actually turn it to airplane mode so that no one can reach me anyway. I can't see GPS+LTE battery life being long enough for any meaningful ride, so you're turning off the LTE and having it on airplane mode anyway, which is back to what I am doing.