Or, perhaps there is an initial period of a year or so where the student is not yet doing a PhD, but is trying to produce results nevertheless. Saw that in Surrey, but can't remember how long students had after defending their PhD proposal and being promoted to PhD student.
Ultimately the goal of a PhD is to learn to be a researcher and to produce a good new research result. If you can do that in three years why wait around another two or more for the sake of it?
I had a colleague who did a PhD in two years in Austria. In that time he got two top-tier papers published. If you’re getting multiple papers into top-tier venues then surely you’ve past the test? You obviously can do research and you obviously are producing good results as judged by a wide group of peers.
Why does the US drag it out so much?
I’ll tell you why - US PhD students also only spend about three years on their PhD. They spend the rest of their time doing masters-level taught classes, teaching (!) and working on their advisors’ projects instead of their own!
I definitely don't like the trend towards the 4 year PhD with taught courses in year 1 though - I had enough time in 3 years to mess around on side projects and other things I didn't need, but which were fun and interesting, even if irrelevant. When you add the inevitable consulting and startup advice on the side, it seems to me 3 years should really be the upper bound, rather than extending the process any further.