From experience with FitBit that kind of GPS usage absolutely eats phone battery. Also, phones are not the most accurate things with GPS. I would have weird meandering paths and cut corners everywhere since it piggybacked off the phone. And this was nearly ideal, dead flat, open area, I can’t imagine how wonky it would have gone with bridges and tunnels and such.

The Garmin on the other hand is so absurdly accurate that I can tell where in the lane I rode even under bridges and through short tunnels, and it will keep that accuracy going literally all day without any battery concerns. I really only need about 16 continuous hours of battery at the most for the riding I do right now, although my wife has been trying to talk me into trying bike packing where a couple days of battery might be useful.

Phone service, I kind of agree can be tethered from a phone (actually, thats exactly how my bike computer does it for live tracking and emergency alerts if I crash). I’m not that fussed about my phone’s weight, so I just stick the phone in a jersey pocket and kinda forget it’s there. The human body makes for a decent enough shock absorber that the vibrations that kill phone cameras on handle bars don’t really get to your phone in a pocket.