ExoticSenta
Well the harem supporters were probably those who were not satisfied with the development of romance between mc and olive. They probably wanted her dead (understandable) and mc going on a destructive path to save her and finding another love along the way. Let's be honest the romance here lacked proper development and almost forced (even if later supported by plot). Proper readers are afraid author will probably mess up harem and tank the whole story (which is quite possible). Dynamics of a harem are preferred in such multiverse fics simply because the interaction and viewpoints of different females with different outlook and upbringing adds different perspectives and helps in overall character growth and more importantly plot development. Personally I'm all for a small (even I hate pokemon harems) harem of girls who act similar to friends/family. Besides the mc is technically immortal and a god so it ridiculous to expect him to follow mortal laws and morality he needs to adapt to his new position and accept it and be true to himself (it is his choice whether he really wants more girls and it also depends on whether olivia will allow it). But now the mc needs a stable emotional connection with more than just one person or else he might lose last bit of his humanity btw things are looking. It seems more of a necessity than wish at this point unless he develops and improves his empathy. The more bonds he has less chance he has of becoming a machine.
Harems are hard to manage without making the characters feel two-dimensional because now you have a whole new character you need to pay attention to and make sure they have screen time without sacrificing the other characters. Otherwise it just looks like you are neglecting one over the other leading to other problems. It's popular in writing mostly because those stories don't care about plot or anything they just do a s*x scene every few chapters because that's easier than maintaining actually interesting relationships but it slows down the story significantly while not actually adding anything.