Well since I have a 10 year old holland lop, and though old with some health issues, is still ticking along, I would say that statement isn't entirely accurate. It might be what that vet has experienced, due to the various health issues rabbits can have and sometimes from the poor care that they receive, but it doesn't mean that an individual rabbit can't live much longer if well cared for and it remains relatively healthy throughout it's life. In fact some rabbits have been known to live much longer, even to the age of 15-18 years. Many times you will find that people may not think rabbits live very long because they are not often understood very well and as a result don't end up living as long as they possibly could, as well as the fragility of their nature at times can shorten their life span.
Hopefully the head xrays will help the vet determine what exactly is going on. It may be a respiratory infection that has gone untreated due to the misdiagnosis by the vet, or it could possibly be something else as you have mentioned. Dental related is certainly possible as respiratory infections can certainly be connected to a dental infection, or even overgrown tooth roots impacting on the nasal cavity. Tumors and thymoma are also possible causes.
One thing that may be helpful if there is any difficulty determining what is going on with your rabbit and how to treat, is having your vet consult with a rabbit specialist. Some of the best rabbit vets reside in the UK with a wealth of knowledge in treating rabbits, and they can be easily contacted and consulted with from all over the world. Because of this training and experience, it can sometimes make diagnosis and treatment more accurate and cost effective as well, as the more quickly an illness is accurately diagnosed and correctly treated, the less unnecessary testing is done and the less it is dragged out. It's what I would do if I wasn't sure my rabbit was getting the correct diagnosis and treatment.
If you haven't already, you may want to consider getting the nebulizer as this can help ease the breathing difficulties. Certain antibiotics can also be administered this way so that they will directly get to the infection in the airway, if that is indeed the cause of your rabbits respiratory problems.
I hope you will soon be able to get to the bottom of this and that your rabbit will get the appropriate treatment and recover well.
http://wildpro.twycrosszoo.org/S/00dis/Bacterial/RhinitisRabbits.htm
http://www.sawneeanimalclinic.com/downloads/rhinitis_and_sinusitis_in_rabbits.pdf
http://www.vetsonline.com/publicati.../n-38-12/respiratory-diseases-in-rabbits.html