They don’t stay whiter for longer...they stay white full stop. Water based is meant to harden with age and, as I mentioned, Bedec is very hard due to its enamel. While I agree that it’s still not there as far as ultimate flow and ease of OB it is 80% of the way but with so many more benefits such as non yellowing, no smell, fast drying so less dust gets stuck.
"Water based is meant to harden with age"
Oil based paint easily passes the fingernail scratch test after a month. It cures much, much more slowly than WB. My GF applied Dulux trade WB eggshell over the yellowed DT OB eggshell about 3 years ago. I can still scratch it off with my fingernails.
Granted, she didn't sand it to key it but it is far less durable. Three years later it becomes soft when the shower is running and the extractor is turned off.
I haven't tried the paint that you recommend, it may well be a cut above the rest.
Last week, I had to help a cabinet maker fit the units that I had painted in DT OB eggshell. Whilst on site, he asked me to hit the skirting/architraves in the rest of the room. He handed me a tin of Leyland WB eggshell. If I dragged the brush back more than 12 inches, in an attempt to maintain uniform tramlines, the paint started to become gloopy and drag.
It looked pretty awful. I was embarrassed. Had it been my job I would not have charged the client. I have no idea what it looked like when dry, probably not as bad as I feared.
I can paint a sheet of 8' by 4' and have constant (read: parallel) tramlines running along the longest length if I use OB. I do not believe that is possible using WB only.