The problem with assimilation is the us vs them mentality being propagated by both sides and further reinforced by the Dalish's tales of racial supremacy and purity. Once that mentality can be abolished (either by eradicating the Dalish or watering down their culture), human and elf relations can progress past the point of fear and hatred.
Another problem with assimilation is recognizing when it even has occurred- especially with the fantasy genetics of the elves.
One of the strongest indicators of assimilation by groups or individuals into a majority is the attitudes toward sexual relations and, well, breeding. Entire cultures have dedicated ungodly amounts of energy to separating racial/ethnic groups and the like, to the point that even dating the 'other' group could be lead to death, or worse, for those involved. Without dismissing the great swarths of categories of 'acceptable' sexual relations that reflect power imbalance rather than integration (such as antebellum American attitudes towards blacks and whites: a white man could sleep with a black woman, but not vice versa), a general rule of societies is that if it's socially permissible to sleep and breed with them, acceptance (if not approval) and assimilation (if not equal) are well on their way.
In the real world, the proof of this comes in the blending of otherwise distinct ethnic/racial features. The most common for Americans would be most African-Americans, who have skin tones significantly lighter than what is often found in Western and Central Africa where ancestors came from.
In Thedas, though... this doesn't exist. Not for elves. Any union of an elf and human will only produce humans. Any elf or group of elves who succedes in assimilation to the point that marriage is acceptable will ultimately vanish from the elven gene pool in future generations because the assimilated as much as they do. It was... Ionia, I think? The female elf in the Human Noble origin? She's an excellent example of the demographic dead in that assimilation is for the phenotype: being accepted by a noble family, and potentially to the point of the M!Human!Warden reciprocating interest. She's almost the ideal of what asssimilation would allow... but any child she and the Warden could have ever held, would have been human. The only way to preserve the elven phenotype over the long term for any assimilated group is to deliberately obstruct assimilation with the humans and maintain breeding programs. Since the only ones particularly interested in this are the Dalish and City Elf elders trying to preserve the racial identity of elvishness...
So in Thedas, when people say there's been no change or assimilation for the city elves, there's no way to prove that and it's highly suspect. All city elves are elves who, for whatever reason, have never had an ancestor who assimilated to the point that intercourse produced children. They are the descendents of those who haven't assimilated or been assimilated. The elven bloodlines that did assimilate, to the point that they could and did have reproductive relations with humans, are simply now human and don't get tallied.