I agree that completing a PhD under the time originally agreed may not be good, as you lose some of the learning opportunities that come with the apprenticeship (yes, it is) program.

However, taking more time than the standard length, whatever it is depending on the university or country where you are pursuing the title, is also something universities and specially PIs should be actively avoiding.

Maybe I have this view because I got mine in NL, where a PhD is a job with a defined length (4 years) and if you go over it, you don't get paid. So yes, it is an apprenticeship, but you should not be doing work for free in any case. Becoming an expert and the (relative) independence on how to do your research are of course selling points of the PhD, but a job is a job.