If it were solely to detect proton decay, yes, I agree it would be very impressive. The reality is a bit more prosaic though, as the article suggests in passing

If it were solely to detect proton decay, yes, I agree it would be very impressive. The reality is a bit more prosaic though, as the article suggests in passing: "In the meantime, a Nobel Prize has been won for a different discovery in the cathedral-esque water tank pertaining to particles called neutrinos." Hyper-K is a dual-purpose detector, a neutrino detector as well as a proton decay detector. Substantial part of Hyper-K funding should be interpreted as investment to a neutrino detector.