I feel it's more that Apple makes a better smartwatch for people who exercise but don't need all the functionality of a Garmin watch. Like you said, Garmin occupies a niche market for hardcore fitness enthusiasts. That is its strength (it doesn't seem like Apple will be able to attack this niche market anytime soon), it is also Garmin's weakness (because they will have a harder time trying to appeal to people outside of this niche segment).
For example, Garmin watches may sport longer battery life, but it sure seems like the supposed lack of battery life hasn't impact sales of the Apple Watch one bit. If anything, it seems like Apple is comfortable maintaining an 18-hour battery life threshold for the watch and channel excess juice into other features such as AoD and extra sensors. This tells you that battery life by itself is not a sufficient selling point for your product (ask Fitbit how that turned out).
This is admittedly cheating, but the Apple Watch integrates with Lumihealth, a collaboration between Apple and my country's health authority which lets me earn points and rewards when I hit certain fitness milestones. It's this gamification aspect that gives me that little added push to go jogging as often as my schedule will allow.
Learn how you can track your daily progress with LumiHealth.
www.lumihealth.sg
The Apple Watch has a wider variety of watch faces (even if you can't submit third party faces). So I can have Siri watch face active during the day while I am at work, then switch to a modular watch face when I want to exercise after work. It's more versatile that way.
What will happen, I suspect, is that as the Ultra Apple Watch goes on to gain additional fitness-tracking features, it will slowly but surely steal existing Garmin watch users who were already on the fringe.