Endgame should be challenging - that's the point (from a casual player)

[Mawlzy]

That's super easy.
First of all: increased HP and defensive stats will get you some wiggle room.
Secondly: SE can set the vuln stack-flag to 0 or re-blance the numbers at any time, if they feel the content is no longer viable.

That being said: people that crave challenge have more than enough to do. I see no reason to up the difficulty of the remaining content to savage level as well. Not every gamer plays videogames for challenge reasons. Many just want to relax after a stressful work day.
One playstyle isn't less valid than the other.

Yeah the last thing I'd want is any more hard content, particularly that gates the story.

I've advocated in other threads for making key content dual-track. You simply cannot keep everyone of all skill and interest levels happy, but as you say, there are many ways to make fights easier for those who have quite enough "challenges" from 9-to-5.