It is all about bringing the aircraft to a stop safely in the landing distance.
Ask your "Captain" how he would handle the situation if he had to land at the same airport, but in the opposite direction; would he cross the piano keys at 50 feet and then float for 1000 metres - not such a good idea on a 2000 metre runway.
If you followed the glideslope and PAPI why would he think you were not on the correct profile?
I have landed on lots of sloping runways (although not in transport category aircraft) and none of them had PAPI or VASIS, and the technique you have described worked perfectly.
I would prefer to fly with you than him.