And it's not like Apple isn't aware of the dangers, either; they used top-byte tagging on the 68000, and it took them a lot of time and effort to migrate away from that to support 16+ MB systems.
IIRC the issue was that 68000 would transparently mask the top bits, so code could get away with avoiding the explicit masking, which of course breaks when you move to a larger address space.
More modern cpus enforce specific bit patterns for the MSBs, so broken code would be caught immediately.
