I agree that there are already many ways to DoS the system-- for example, the age-old fork bomb. But that is not a good reason to add more flaws. People are working on ways to fix the old flaws, such as cgroups.

I don't think a ulimit would be very effective here at preventing denial-of-service. Let's say I set it to 100... I can just have my 100 children each spawn and hold on to 100 children of their own, and so on and so forth. If I just go with a bigger process ID space all these headaches go away, plus existing software works without modification.

32 bits is still too small. I wouldn't be comfortable relying on the size of the PID space to avoid collisions until we made it 128 bits or so. I think you're still seriously overestimating the danger of a DoS here: whatever limits apply to forked processes can apply to process handles. Whatever mitigates fork bombs will also mitigate handle-based attacks.

The advantages of process handles outweigh this small risk.

In what scenario would you run out of 64 bit PIDs? How many per second for how many centuries?

It's not a matter of running out of PIDs: it's about the probability of accidental collision.

A workable limit is trivial, how about 100 zombie process IDs per user.