There's a lot of reasons I think time-travel would be a bad idea. The fact that the characters are aging in real time, especially Noodle, is really special to a lot of fans who grew up with the band. Since they tend to make a lot of sociopolitical statements with their albums it makes sense for them to stay in the modern era. Gorillaz lore is already convoluted enough, and time-travel tends to just complicate things further. It would likely retcon a lot of canon elements that Jamie and Damon probably forgot about, but that the fans are attached to. There's something almost kind of judgemental in the idea that they would send the characters back, almost like they feel the need to fix something and are taking the easy route to do it. It also just feels out of character, too. Gorillaz have done a lot of outlandish shit, but it was all cartoonish shenanigans and never so unapologetically sci-fi. And if you ask me, time-travel just feels so tired and over-done lately.