The point is that a 30% failure rate is less than the 50% discount so you only end up saving 20%, with the spares included in the purchase.

But if 20-30% failed in the first few months of an expected 5+ year life cycle, that's indicative of a >50% failure rate in the medium to long term. You'll probably come out a loser in monitor-years/dollar.

That does not necessarily follow. See the "bathtub curve".

See also "time value of money" - if you don't have to buy the spares until they fail, all that extra money is worth something to you.

There is a reason Toyota and co don't have 6months of parts on hand - and it isn't due to shortage of shelf space