What you provided isn't (just) maths, it's a modelling assertion: that failures are independent from part to part, and independent from day to day. I'm skeptic

What you provided isn't (just) maths, it's a modelling assertion: that failures are independent from part to part, and independent from day to day. I'm skeptical about your first assertion -- think of the effect of a power surge. I don't agree with your second assertion at all, from my experience of cheap devices that fail not long after their warranty expires.