Your numbers don't line up, if you are spending 5k in cloud costs, and on prem is 1/3 of cloud. At 48 month replacement cycle, 1/3 of 5k * 48 months is 80k. So it is 80k vs 5k a month for 48 months.
I think the primary reason that people over fixate on the cloud is that they can't do math. So renting is a hedge.