Honestly the part nobody brings up is what those half-life inserts are actually doing to your surface finish and your dimensions on the roughing pass, because if the insert is breaking down faster it's not just a cost per edge thing. A worn insert starts pushing the material instead of cutting it, so your tool pressure goes up, and on 4140 that can mean the part is moving around in the chuck or the turret is taking a beating you won't see until something lets go. Your lead hand is doing the math on insert cost versus cycle time and that math works fine on paper, but he's not the one standing there when a rougher lets go and scraps the part or worse. I'd start tracking your insert wear pattern instead of just counting how many you burn, because if you're getting even flank wear at 40% more feed then yeah he's right, but if you're chipping edges or getting thermal cracks you're just trading cheap inserts for expensive spindle repairs. And tbh the fact that you're the one swapping them means your time is part of that cost too, even if nobody writes it down.

I saw a thread on a bushcraft forum where a guy did almost the same test, carried a waterproof notepad for years, then switched to his phone for a month and never went back. His big thing was losing the pen too, said he went through like 6 in a single summer because of clips catching on stuff. What stuck with me was a comment from some old timer who said the notebook was never really about the paper, it was about not having a screen between him and the person talking. That part matches what you said about the job site thing. For me the tiebreaker is the camera, half my notes now are just photos of labels and serial numbers instead of me trying to write them down in the rain. The only time paper wins is cold, my phone dies out in the field in like 20 minutes when it's below freezing but a pen still writes fine.