I understand your concern but it is hard to put an hourly life on a bearing when it has different environments to work in. One pilot may fly easy and in cool weather and another may fly in hot climates like a bat out of hell. If DJI said 100 hours life, someone with 98 hours and a failure would raise all kinds of hell. They couldn't possibly state every single thing that could happen to a Phantom in life. Re...how many hours can the Phantom fly in the sun before uv affects the shell.....list every cleaning material not to use on Phantom. ...how hard is too hard to hit something. ...etc. They won't put themselves out for such liability. They have enough as it is. Does your car manual state how long your wheel bearings will last?
I just meant that dji must have collected a lot of experience flying Phantoms and bearing are a critical component so they could at least say something in the manual about. "bearings are sealed and are not designed to be serviced" or "operational life can vary depending on air temperature, humidity, dust and salt spay etc. as well as your flying habits". Treat it gently and it should last for 150 hrs of whatever . . drive hard and it won't. . . If I was editing the manual I'd at least add a similar statement. Instead their documentation lacks a lot.
My car manual at least tells me when and what I can service and I expect it to last longer than the warranty mileage. I get it's a "toy" but dji knows much of its whole lineup of machines are be purchased as professional tools. Owners should have some clue as to what "life" to expect.