They're both massive changes to the status quo. There may be more Cardassians than Romulans left, but they've still gone from a major galactic power to a zero in a heartbeat. And what happened after this massive change? Nothing.Ok there's no way you can see that as directly comparable.
First of all, Cardassia was not wiped off the face of the galaxy. The Cardassian homeworld is still intact and Cardassians will still live there. Second of all, as I've explained before, Cardassia was DS9's baby. The writers of DS9 spent 7 years fleshing out that whole region of space, they earned the right to bring their stories to a meaningfulconclusion. Now if in the DS9 finale we saw Troi coming to visit DS9 in a shuttle and it exploded, that would be a different matter, albeit hilarious.
"Earned the right" is still a complete joke. Star Trek was dead, JJ Abrams's team brought it back successfully, with a hugely entertaining movie. In doing so, they've most certainly earned the right to do what they want! To say otherwise is elitist crap.