Let's say your high quality supply's yearly failure rate is 100 times less than the cheap ones

The probability of at least a single failure is 1-(1-r)^70.

This is quite high even w/out considering the higher quality of the one supply.

The probability of all 70 going down is

r^70 which is absurdly low.

Let's say r = 0.05 or one failed supply every 20 in a year.

1-(1-r)^70 = 97% r^70 < 1E-91

The high quality supply has r = 0.0005, in between no failure and all failing. If you code can handle node failure, very many, cheaper supplies appears to be more robust.

(Assuming uncorrelated events. YMMV)