I used the phrase in the last post, and I think people get what I am saying and roughly assent to it. But our impression is still often strongly conditioned by what we hear and think we know. Let me give a common example. There is a prevailing myth that keeps showing up that Appalachian English is closer to, and a preserved version of the Elizabethan English of Shakespeare, and perhaps even farther back, to Chaucer. I seems that every ten years since 1860 some person with a degree stumbled upon a feller in West Virginia saying "afeared" or a woman in Tennessee saying "britches" and leaped to the conclusion that there is something purer and older about it. I will note in fairness to such folks that the accent in Appalachia, while not identical to older forms of English, is likely the closest thing we have. The Great Vowel Shift primarily occurred in the 15th C, but there were waves of it up until 1700. Yet the northern dialects and certainly the Scottish were much less affected throughout that whole time. The English Borderers and the Scots-Irish were the primary settlers of Appalachia, but there were always some African-Americans and Germans in there bending the language in unpredictable ways as well. All off those were less affected by the vowel shift, all waves.
But isolated languages can go through more change, not less. Peoples trading up and down the coast using a half-dozen regional dialects will tend to draw closer to each other, especially in vocabulary, not get farther apart. There is a counterforce that the traders also have contact with people speaking other languages, which will pull them farther from the dialect of their grandfathers, mostly in vocabulary, but somewhat in syntax. The isolated (mountain or island) folks won't hear that as much. In the main, isolated groups tend to get more and more distinct, as in Papua New Guinea or the Caucasus. They do preserve older forms - but each valley preserves different older forms. So in Appalachia there is the nice old word "poke," meaning pouch or pocket (clear cognates) as in not buying a pig in a poke - not buying something you haven't looked at closely.
The coastal people also preserved older forms that the mountain people dropped. Like what, you say? Like everything else. All dialects preserve some old things and drop others. The 90% majority doesn't notice - it's just normal talk. Only when they encounter the isolated minority do they notice.