To clarify, we are asking why would a 2015 car require a whole new chassis in 2021??
This car may be registered as a 2015 but I suspect it may have been built well before 2015, just a guess
Looking around in the M3W market place it seems that some of the early speculative buy one's are now appearing with 200 miles or so on them, so may well be in need of a new frame as some are early one's....quite a nice little earner doubling the price but maybe not such a bargain really for the unsuspecting innocent purchaser.
Buyer be aware..