Yes, I agree, I never buy these things new. You are right, it's an exponential drop from 0-4 years, followed by a linear drop from 4-10 years.
But $30k in 1.5 y is a lot. That's like 26%. The E60 were certainly bad too. I picked up mine, like new and loaded, 42,000 mi, 2.5 y old, $42k. I think the sticker was $99k.
Bought my Jul-06 build in late 2019 (3.2yrs old and 25K miles) for $36K
and spent 4 fantastic years (and going). Even if I get $25K for it now, it well
worth it. Probably spent $5K on maintenance (rod bearings, clutch etc).
I wonder if the uber priced M6 GC will depreciate at an even higher rate or hold more value as it is very desirable?
I also think the substantial F10 LCI changes (steering wheel, LED lights, suspension changes, availability of the competition package) didn't help the '13 F10 M5 resale values. I am not sure if the E60 had as pronounced changes during its life cycle to damage the earlier model resale values.