To answer some questions, as well as produce new insight...
Spoiler for Doomsday Landscape Aftermath:
And Johann's Death: The doomsday landscape is the scenario with doom coming out of any possible outcome. The landscape that Johann was trying to show to Tenma was the loneliness that would come from Tenma shooting Johann. In the Doomsday Scene, only Tenma and Johann were there, as if they were the only two people left in the world. Just as Anna and Johann were the only two people left in the world, and Anna was asked to shoot Johann. Another problem with Tenma shooting Johann is that if Tenma did shoot Johann, then Johann would probably be able to complete his goal. In many of his manipulations, he has surrounded everyone who knew a certain person's past.(a.k.a. General Wolfe) If you kill everyone that you know, then that in itself is the perfect suicide, pure of loneliness. According to Johann, as well as the events within Kinderheim 511, even if you lose your memories, the memories of you will be kept with the people that know you. Therefore, if Tenma killed Johann, then all the people that Johann had isolated would be completely lonely, as Johann was their last acquaintance.
In addition, Tenma would also be lonely as Johann is living proof that Tenma is not a killer. This is hinted at the very end as well when Johann theoretically jumps out the hospital window. Because Johann tried to erase the memory of himself by erasing all the people who knew him, Johann tried become imaginary to the rest of the world. At the scene where Johann is standing on top of the deserted building and wants Nina to shoot him, Johann just wanted to be as imaginary a being as possible before he died. But this of course is his depression, as noted earlier by Martin's quote, "No one truly wants to die." If Johann wanted to truly produce a perfect suicide, then Johann would have killed Bonaparte, Nina, Tenma, Roberto, and all the kids from Kinderheim 511 (which he kinda tried already in the giant massacre).
Therefore, Tenma would be left alone without Johann, since Johann is his justification for why Tenma is not a killer. As noted in the newspaper clippings collected by Eva, people still think that the whole case is imaginary, just as Johann would have wanted it to be. Again, one of the biggets morals in the story, "The only living proof that you existed resides in the memories of others."
Johann had tried to be imaginary in the first place as he has always believed that it was he who was supposed to go to the Red Rose Mansion. Because the mother had tried to hide the fact that they were different, and show the way that the mother wanted to treat them equally, she dressed them as identical twins, both as girls. Therefore, Johann became obsessed in the idea that the mother had chosen one of them, and wanted Anna to feel that she was the loved one. Johann chose to uptake Anna's memories and suffer for both of them. Just as Poppe's book describes the monster with no name, Johann decides in desperation to follow that character. Ever since, he has always thought of himself as "unwanted", or "imaginary" and has strived to procure the world which would produce the most happiness for Anna and Nina. Johann decides to erase his existence forever, and probably wanted to erase Nina as well, to ensure that the experiment had never happened.
The procurement of the doomsday landscape was spurred by Johann's disagreement with Tenma when Tenma believes that "All lives are equal." In the terms of equal life, it should have been possible for Johann's mother to not be able to choose a child, but instead, she had chosen one to stay and one to leave. If their lives were equal, then why was it that one was to go, and one was to stay? ...one was to live, and one was to die? What was it that made one better than the other? What makes it worth saving one life over another? Johann explicitly conveyed this message to Tenma, in the beginning of the series as well as the end, as there was a scenario with the boy dying, or Johann being shot by Tenma.
In creating the monster in Kindeheim 511, the staff and people (many of them) worked to undo the creation of a monster. In doing so, they also themselves created a monster, killing everyone who knew about the experiment, and producing a mass suicide, so to speak, as the one in Kinderheim 511 in which everyone that knows anything of the experiment will die.
Ahh, this is just a subject that could go on forever. What deep philosophy that can come from this great series. Anyway, this is too much of my own opinion. I don't wish for everyone to completely adopt my view, but I hope it helps!