I agree wholeheartedly with 
@criver .
Being able to revive people who have long died 
does limit the engagement the viewer has with the emotional aftermath of a death of a beloved character. The reason i teared up when Gugu died was 
because he was not gonna come back and seeing the effect it had on the people around him and the MC. This death gave meaning to Guru and his story. MC had to carry on with his will and that strenghtened him during his further journey. 
Reviving him after 200 years is a horrible idea because it cheapens the earlier described experience in hindsight. His death becomes entirely irrelevant because MC can just revive whoever he wants now under some 'arbitrary' conditions. The biggest glaring issue with this is who he can and can't revive, with the greatest inconsistency being the princess. For a revival to take place the "spirit" still needs to linger around to inhabit the body copy. There is no reason why the princess would not linger around her corpse to be revived. She is the princess of an entire country, which is on the brink of destruction and her life long friend finally became friendly with her again. Out of all the people who died so far she has the biggest reason to still linger around, and you're telling me she passed on in a second? That seems hardly fitting.
This manga started fantastically, being one of my favorites, with a compelling MC and story to tell about an object evolving over time, influenced by his environment and the people who interact with him. This pretty much became moot when this arc started, and that is when in my opinion the story took a huge plunge towards the negative. Reintroducing Tonari, who was THE mc in the whole prison island arc, after 40 years of her experiencing the world, just to kill her off in the same effin chapter left a gigantic sour taste in my mouth. The whole defense corps debacle and the 200 year time skip after that didnt make it better. Fushi turned from a young adventuring boy evolving and influenced by his surroundings to a godlike necromancer who can summon entire cities from the palms of his hands in the span of 1 arc. 
In conclusion, the people he summoned now have no reason to even exist at this point. 200 years have passed, their entire purpose was to watch over and help fushi with their forms which gave him power. 
Not a single one of the ghost people that were following him did it to be revived by him. The only reason they followed him was because they loved him and wanted to watch over him and see the changes he would bring towards the world. I doubt they even want to continue to live around at this point, because every single person they knew is long gone and their cities turned to dust. The author can NOT keep these revived people around, that would be terrible storytelling. At most they can complete this current objective and then pass on and join the eternal cycle, which is what im assuming is gonna happen.