Fact is there already IS a fair few minor hints as to JK losing health - it might not be all that obvious but it does happen, secondly the animating of him getting more cut up and injured is not an easy thing to animate - or implement for that matter; in the matter of Roars it's simply a degrading of the texture showing off more dmg than normal, if you wanted to do the same to a 3d seethrough object like JK, we're talking a whole other dimension of animation.
Let me give you an example; We have a giant cube with a crown, this is an animated character, but with a static layout and design, to implement damage showing on him you'd have to switch his model several times during the battle with one who's more injured than the last, making the fight incredibly laggy as new data would be constantly refreshed, and so many more variables would be added for the client to keep an eye on. I'm not saying it's impossible to do - but it's a large task for a tier 2 boss that's not all that bad as is.
tl;dr - They could, it's alot of work on old content which takes focus away from the new, so they probably won't.
Yes u r right...But what do u think about my idea with stages for JK ? I think thats good...To add some visible damage like Roars have ...And fo sure when he is regen himself wounds will disappear