With proper sandboxing, it is safer than keeping deprecated, proven-unsafe APIs, and not even being able to fix bugs because some applications depend on them. In the worst-case scenario, you can restart the sandbox from a known-good snapshot.
EDIT: I realize we may be imagining different scenarios. I was thinking about the example given, of running original DOS programs in modern computers.
There seems to be a leap you've taken there. I don't think you've demonstrated that an old API is unsafe by virtue of being old. A platform company not wanting to maintain an API is not the same as that API being dangerous. This is the fallacy that some of the big guys have taken lately, to remove perfectly working stuff simply to be jerks.
EDIT: I realize we may be imagining different scenarios. I was thinking about the example given, of running original DOS programs in modern computers.