Yeah, I know nothing as well but I have done a TON of research since this post. I'm actually looking at 40,000 miles but $60k in price which changes the equation a little. Your rationale aligns with my thought for early 2021 value of MX, but I baked in a +5k adder since the SUV is more expensive. Probably see $45k on MX by 2021.
MX 75D with AP2 currently costs 84.5 new (before tax incentives). If I do 25% depreciation in Year 1 and 10% in year 2, that gives me a value of $55k 2 years from now. I'm not sure if the Year 1 depreciation is correct - but 10% of that is tax incentives and not true price depreciation for the initial buyer, so I'm now hoping to get a MX 75D with less than 40k miles for less than $60k inside of 2 years.
Based on the same percentage assumptions, might even be able to get a 100D for $65k in 2 years.
That's my thought at least. We'll see leases ending and Model Y pre-orders and maybe early deliveries in these time frames which will probably push down the current MX's as well as new features in the technology.