If you run a buggy program on a modern OS, it won't crash the system or impact other processes. If you run a buggy program on DOS, it will write to random physical addresses, probably clobbering the state of other processes and of DOS.

Modern OSes can run arbitrary binaries, but they can pretty much run arbitrary non-adversarial binaries - problematic binaries have to be intentionally written to exploit the system (as opposed to DOS, where non-problematic binaries had to be intentionally written to not break the system).

It's a dramatic improvement.