On the surface, I agree: there's an interesting problem that's worth solving, and a purely artificial limit is forcing people to do a bang-up job at solving it.

But if you dig a bit deeper, I can see two counter-arguments:

1. The real risk -- by which I mean "the risk I have most often observed in the wild" -- is that a Ph.D expands to fill the time it's given, without ever wrapping up and producing a publishable result. This happens so often that it's practically expected in some places.

2. Having a deadline, oddly enough, also serves as a catalyst for birthing an idea... for "pinching it off" as the expression goes. At some point you have to stop planning and start executing. You can see the deadline as a forcing function.

Ph.Ds are needlessly traumatic and procedural in many ways, but I'm no longer sure that hard deadlines are a net negative.