Saturday morning cartoons isn't nostalgia for me nor was it a trend. Shit was always awesome. Batman Begins, X-Men TAS, Spider-Man, Reboot, Superman TAS, Pinky and the Brain, Sonic Saturday Morning, Looney Toons, Freakazoid, The Tick;etc. were objectively good. And while they weren't on Saturday, they were on Sunday, shows like Ren and Stimpy and Rugrats were legit too. DBZ was also on Saturdays.
Labeling everything old as "nostalgia" is probably a worse trend.
It's also weird to label people loving Saturday morning cartoons as "recent" or a "trend". Adults in my age bracket have always talked fondly of Saturday morning cartoons. This doesn't mean they had no legitimacy.