The catch here is some of us don't want multiple devices. We want one that's always with us. I would go out of my mind if I was constantly having to swap what I was wearing and tracking on multiple different platforms. I don't want multiple ecosystems. You completely ignore the value of working in a single ecosystem. I also own garmin accessories (chest strap, running dynamics pod) along with a garmin bike computer (cadence sensor, speed sensor and chest strap for that too). It all talks.
How do you get your Teric dive data into Garmin? You can't. But I can put my Garmin dive data into Subsurface.
I'm not ignoring all that. To ME, I don't really see the value of having my dive data in the same "ecosystem" as any of the other data that a fitness watch would track. Even if I could have heart rate info in my dive data, why would I really care? What is really important is gas consumption and heart rate is really only another way of looking at that (I think) - and less good, really, than just having the consumption data.
Yes, you can put your Descent data into Subsurface. But, that is actually a negative point, to me. Not that you CAN put it in, but HOW you have to do it. You cannot download directly from the Descent into Subsurface. And, that is ONLY because Garmin dictated to the Subsurface developers to NOT allow it. Garmin required it to be done by downloading to Garmin and then exporting that to import into Subsurface.
I really like how easy it is to download via BT from my Predator or Teric directly into Subsurface on my phone, while I'm still out on the boat, usually. And then it is just "there" in Subsurface on my laptop when I get home, without any extra steps.
I don't see where having my dive data going through the cloud first and adding the extra steps required to get it into Subsurface would actually be BETTER for me, in any way.
Lastly, if it would drive you out of your mind to take off your Fenix and put on a Teric every time before you jump in the water for a dive, I really think this discussion is moot. I can't help you.
The value in having one ecosystem is only (to me) when it's one ecosystem for a substantially similar body of activity. Scuba diving is completely different (to me) than running or hiking or moutaineering or all those other things. Scuba is not a "fitness" activity, to me. It's a relaxation activity. Trying to cram those two things into one ecosystem is (to me) like saying I want my commuter car and my weekend camper hauler to be powered by the same "fuel". That is silly. Get an EV for commuting, and a diesel pickup for hauling your camper. Use the best tools for the jobs you're doing. In this example, I reckon a Teric is better for diving and a Fenix is better for fitness tracking. To me.