Damian was there out of the blue as well. Most legacy characters are like that.
The problem with this is that they don't see it like that. They see it like this. Superdad and Jon's origin story-read convergence, lois and clark. You are basically caught up. It can function like one effectively. They think that was the origin and this, the current status quo.Unforseen change has been part of the narrative since birthright. That's a whole different issue. But, Rebirth's promise was stability(anti reboot) , history.. Etc. Which meant expectation of this status quo lasting a long while .People got suckered in. It's not like people knew this was going to happen and They were going to return to form.
Again age up is forgivable,if bendis actually got Jon's voice correctly and superparents didn't become comicbook version of goku all of a sudden. Jon just lack any personality now. Before he was goofy, energetic, spunky.. Etc. The stories were like that of a shonen protagonist exploring his world.which i must admit it still might be,If a certain theory is true.