I just finished Chapter 6 last night, and I like how the game ends. Have not started the Epilogue yet.
I think it is totally fine that the story kills off Arthur in the end. Usually in open world games, I have zero desire to go back and do side missions after the main story is done. That somehow always felt weird to me, to go "back" after the story is over. Something about the narrative ending, breaks the game world for me. Which is fine.
So I actually prefer it this way.
Also, it didn't bother me at all that Chapter 6 had Arthur be sick and weak. In fact, to me it helped the narrative and made his side missions where he tries to redeem himself make sense. Great choice by Rockstar in my opinion.