I took away something completely different from reading the graphic novel.
For me, it's a story about failure. The heroes fail, pure and simple. They have one job, and they fumble the ball. And Captain Marvel dies.
And Captain Marvel himself? He's not a hero in this story. He's nothing but a helpless victim. The heroic act has already been done. That happened years earlier in Captain Marvel 34 when he seals the gas canister. This is just a generic fight scene followed by sixty pages of Marvel saying goodbye.
Starlin even sidesteps the very issue he's trying to portray - death by cancer. What he gives us is a nice, clean, anodized version of the process. Marv coughes a few times, but where's the pain? Where's the suffering? Where's the pain of his loved ones as they have to watch him go through it? It's not here. It's absent. He just slips quietly away.
Even the ending is unsatisfying. The reviewer is taken with the notion that Thanos - Marv's greatest foe - is the one to lead him into the light, but that's not what's really going on there. Put simply, Captain Marvel is near death, and has a common near-death experience. The last few pages are nothing more than the hallucinations of an oxygen starved brain.
So basically, Captain Marvel ends his life with his body riddled with disease and his mind addled.
The Death of Captain Marvel is basically a vanity project. Jim Starlin's attempt to work through his grief at his own father's passing by casting his parent in the role of the hero and trying to find some meaning in the loss by reaching for the metaphysical. Therapeutic for Starlin maybe, and maybe some of his readership found some comfort in it too, but the story fails on almost every level for me. There were half a dozen better ways to tell that story. Starlin goes unerringly for all the easy options.
A depressing and ignominious end to a great character.
Mar-Vell deserved better.
You are right, Mar-Vell does deserve better.
Did the heroes fail? They sure did. However the initial failure was Marv's for not telling anyone about his illness until it was too late. When ISAAC scanned him and gave the negative prognosis Marv was likely at the Stage 4/systemic stage where the odds are not in your favor.
The pain was there but it could have been shown more. Marv does get a pain attack and smashes a computer in frustration as he tries to accept that he will be gone.
Rick Jones was severely upset for some time until he sat with Marv and they chatted for awhile.
As to Thanos coming to get Mar-Vell, I do not think that was a dream, I think Death really did send Thanos to come and get Marv and give him one last fight. Cosmic Powers Unlimited #1 has the backup story of Thanos restoring Mar-Vell via the Reality Gem.
Marv states that it was Thanos that helped Marv accept Death and that he was resolved to being dead and that he didn't want a second chance. he also talked Thanos out of his plan to use the gem for warp things so Death would love him. Thanos agrees and sends Marv back to death.
But yes, DoCM was indeed a catharsis for Starlin, there is no getting around it.