Interesting...
"Outside of the Inquisition's inner circle, most people believe Abelas to be part of a delusional sect. Historians have deemed the Sentinels' arms and armors, which were bought back from the Arbor Wilds by Inquisition soldiers, of superb craftmanship, but don't believe any cloth or metal could have survived untarnished as long as Abelas claims." - Abelas, Trivia section on the Wikipedia. It cites World of Thedas vol. 2 as the information source.
"We are trapped. The ones born here do not understand the keenness of what we have lost, or why so many of their elders weep as they enter uthenera. The new ones are faithful to Mythal, but do not understand what she was in her fullness. Without the wise to lead them, they will lose what they should have been.
I will teach them. They must serve. We must prepare for those who cast Mythal down. I shed my name the day I began her service. I shed my new one again, now that she rests. I will only be known by the sorrow that cuts my heart." - Codex entry: Untranslatable Elven Writing
This is very interesting. I think it says several things. Just my take, I'm open to further evidence/ideas.
1. "We are trapped." They are. Their duty to Mythal binds them to the Temple. They're not free to roam the earth, set up their own societies, or interact with the denizens of Thedas as they see fit.
2. "The ones born here do not understand the keenness of what we have lost." They've lost their glory as a society. They've lost a culture vast and ancient. They've lost their god and leader, Mythal, who cared for them. Who aided them. Who taught them. Who defended them against the Evanuris excesses.
They can also be experiencing some effects of the Veil. The Veil can experience "thinness" and have varying results with individuals, reducing or eliminating their conscious connection with the Fade.
The preservative results of the magic of the Temple if any, is unclear. Remarks about cloth and metal's survival hints at preservation as a benefit to existing within the Temple. Maintenance can only go so far in thwarting corrosion and/or wear and tear.
The elves in the Temple do use uthenera. Uthenera wasn't a method for preserving the long life spans of elves, rather it was a method for elves tired of existence to take a break or slowly drift away from the sadness of life. It was also a method for exploring the Fade and gathering knowledge. This makes it unclear at best if they used uthenera for extending their life spans (because Veil) or for alleviating the boredom of existence (Veil mitigated by Temple magic/preservation) stuck in their Temple for thousands of years bound to duty. Boredom can drive a person insane. There still isn't enough evidence to support that uthenera succors immortality, so I lean more towards its use in the Temple to relieve boredom and probably exploration of the Fade.
There is no evidence that Temple elves die of old age. Abelas tells the Inquisitor that their numbers are dwindling due to repeated encounters defending the Temple. "Without the wise to lead them" is reference to Mythal. "The ones born here" indicates that there still exists ones that were not born in the Temple. No elves in the Temple are permitted to wither away in uthenera. They are under a geas to serve Mythal until they are released by Mythal or slain. Abelas says they are awoken to defend the Temple.
3. "We must prepare for those who cast Mythal down". The Evanuris were very powerful. Solas mentions events that created oceans of blood. They either expect the return of the Evanuris eventually because they don't believe they can be trapped or gone forever (very likely) or ... this is a hint that Solas and Mythal working together had already discussed the possibility of the Veil. This knowledge could be in the Well, or passed down word-of-mouth/written stories. It's also possible that Solas left instruction to his forces before he raised the Veil, preparing them for his failure or an unforeseen collapse of the Veil and those forces spoke with those in the Temple some short time after the Veil was raised.