Why not open source the firmware?

When you already have companies like Coros making almost identical copies of your hardware you don't want to make it easier for them by giving them a full look at your firmware as well.

Not to mention the Technical Support nightmare that user-modifiable firmware would cause...

Lawyers. The code likely contains tons of patented stuff and licensed code from other companies that Garmin can't just make openly available even if they wanted to.

Exactly, you only have to go to system-about and scroll down to see all the copyrighted portions of software in the firmware.

Most of the firmware doesn't have any secret sauce, the pool lap bug as an example would be a logic oversight. The closest to secret sauce you get is the detection of laps, v02max, all of which can be (and should already be if the code is well designed) external calls. So the secret sauce can still remain closed sourced. Given how difficult the firmware is to maintain those calls are probably not external and the architecture is poor. For example who cares if the menu is open sourced and Coros or other competitors can see that? Nothing because software doesn't work like that, you can't just copy and paste software and have it work and most of it is nothing special, there is nothing special about the garmin settings menu.

That's not how open source works my friend, garmin would still have the final say what goes in and what doesn't.

That patented code should all be external calls. They wouldn't even have access to the source code of the patented code if they are binaries and the calls would be forced to be external. If those calls are not external now, even if they have access to the source code then the code is shambles and that's why we are seeing so many bugs.

If Coros or other competitor open source their firmware, and their hardware is just as good as Garmin's, Garmin would be left behind in the dust. There's a reason why the vast majority of super computers run open source and not proprietary operating systems and it's because it's just better quality.

All the other stuff should be external calls already or are licenses which you must include by default, even if it's an MIT license, etc.

In order for the open sourcing to make sense, you would need the build toolchain and tools that allow the firmware to be flashed on the watch without digital signature, and the ability to toggle whether the watch allows these insecure versions to be flashed or not. Just making the source public will not solve any of these issues.