You can point to Travis Hafner (shoulder injury) and Mo Vaughn (weight) who were both elite into their early 30s.
I can point to people like Thome, Giambi, and Delgado who hit their peaks in their early 30s, and are still extremely productive into their late 30s (and you can point to steroids as the explanation)
I hear this a lot that people with "old person skills" (which I guess means power and taking a walk), don't age well, but Dunn is 28, and there is absolutely no indication that he should be any worse than he has been.
Depending on the park (and playing time with their glut of outfielders) he may not be as valuable from a fantasy perspective, but I don't see much indication to think his skills will erode at all in the next 2-4 years.
Dunn is better than Abreu right now - and has been for the last two years. Dunn is still in his prime, while it's clear that Abreu is on the decline. They both stink defensively (though Dunn is worse), but Dunn will likely log more time at 1B now than OF, so that should reduce some of the draft on his value.