It does not take in the scenario where newer engines are extremely reliable and older engines have a dramatic drop in the reliability rate.
Your presumptions are not valid.
Overall (millions of hours) there is such a thing as a bath tub rate high during the learning process of what's near perfect and what doesn't work as expected. These kind of problems get high attention and only after enough hours are ETOPs considered. Then there is the mature rate where wear-out rates are kept track of in scheduled maintenance. In the end the rate often goes up when the product line moves to third tier operators with less diligence.
Meanwhile as I said in my earlier post, this problem, as described so far, is a common cause risk aggravated by a much faster and less defined wear-out rate that has little bearing on the ETOPS data assumptions. As such, day to day
inspection results predict the liklihood of any two engines on the same aircraft failing in the same flight.
The fleet mix needs to address this by combinations of engines in and out of the new and changing maintenance schedule. Hence the available engine resource problem until enough "fixed" engines are available to match route structure.
ETOPs is not a protector when you are dealing with dual engine common cause failures since it only assesses the very small probabilities of independent failures