Earlier quoted context omitted.
It works out all right on Windows, which uses exactly the approach I advocate. And you can already DoS the system in myriad ways. If you're still worried: we have ulimits for other resources. We can have a ulimit for this one too.
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…
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.