I agree with your conclusion of increasing the length of support. But your half-life analysis isn't really the right choice. Hard drives tend to have a bath-tub failure rate: https://en.wikipedia.org/wiki/Bathtub_curve
(Also, my hard drives always fail after I put them in the bathtub. Lol.)
E.g.: Mybook Live Duo 8TB: https://productimages.hepsiburada.net/s/12/500/8922723713074...