I've tested different builds already, & getting more HP out of runes after level 40 is far more beneficial, outside of the defiled chalice, nothing one shots you in the game in pve, & the extra ~150 HP you get from those ten levels would be the difference between having 20% or 30% remaining HP, with more stamina though, you get far far more dashes & rolls, that's a lot of iFrames, as well as longer stunlock times. PvP usually relies on your skill as a player more than PvE (stats are important too of course), & with more stamina you get better opportunities to utilize your skill.
That's probably why nobody runs around with 20 END anymore, when Bloodborne first came out people treated END like it was Demon's Souls or Dark Souls, that's why people stuck with 20 END for a few months, but once more people started experimenting, more & more people started realizing how important Stamina is in Bloodborne, 10 levels of HP will give you one more hit of survivability, 10 levels of Stamina gives you 5-7 more dashes.
You can make an argument that a 40 END build with BoM would wreck a 50 HP build with potato Stamina.