I'm assuming you mean 15mm inner width...

No, you won't die. For me, on narrow rims, even 25s with their reduction in tire pressure, feel squishy in turns. I prefer 23s for rims that narrow.

I suspect you're losing a fair amount of aero due to the "lightbulb" shape of a too wide tire on a narrow rim.

My old cross bike has narrow rims and copes just fine with 34-40mm clincher tyres. Down to about 30 psi.

On the road you won't have any difference, off road the difference is significant .

My cross bike came with stock with narrow (13c I think) rims and 35mm tires. I changed them out for Hed Ardennes+ which are on the extreme wide end on the rim spectrum. Above 60 psi or so I couldn't discern a problem with the narrow rims (other than the fact they were super heavy yet always coming out of true) but once I dropped much below 50 psi I could feel the tires flexing in corners on the narrow rims. On the Ardennes I have ridden with less than 30 psi and the tires stay really solid in the corners.

Going in reverse the Hed wheels are an absolute PIA to mount 28 mm tires. I mean you seriously wouldn't want to have to change a flat on the road with them. If the plan is to run 25-28mm I think 15-17mm is the sweet spot in terms of rim width.

Yes, the rims have an internal with of 15mm.

I don't want the handling to be completely marshmallowy, but I'm not concerned about it being particularly sharp, either. It's quite flat where I live, so I'm not doing any aggressive descents with switchbacks or anything of the sort. For the most part, "handling" around here consists of the occasional 90* turn off one long stretch of arrow-straight road onto another long stretch of arrow-straight road. A lot of those roads are chip-sealed, though; I'm willing to sacrifice handling crispness if it will minimize the amount of "buzz" that transfers from the road surface to the chassis.

This is an endurance road bike so aero's not really a concern. The rims are box-section, anyway; DT's old "Open Pro" equivalent. The rolling stock's gonna be as aero as a brick regardless of tire size.

---

"They're made of latex, not nitroglycerin"