Was Rose sucked into oblivion or did she move on like Lexi?
When Silas drank the cure from Katherine she started aging rapidly. If Damon drinks from Elena after bonnie dies won't she start aging really quickly as well?
Julie said everyone who was still on the other side when it collapsed, got sucked into oblivion. So unless she found peace before that and we just didn't see it, then yes. She got sucked in.
Katherine aged rapidly because her real age was over 500 years old, once you lose the cure from your body you start to age up to your real age and since she passed the natural lifespan she died. Whereas Elena had only been a vampire for less than 3 years so she'd only age up to about 21 years old. Its only how long you were a vampire that matters, it doesn't matter how long the cure is apart of you so even 60 years from now if or when Elena loses the cure, she would still only age the 3 years from when she was a vampire.
It wasn't a cop out because that's what happened. Kat didn't start rapidly ageing until after Silas drank the cure from her. Damon will only die if someone drains him of the cure and as long as the cure is in the vampire's system they won't age rapidly, just normally.