Personally, although my group seems to like it easy peasy, (no idea why), i love a challenge. My friends complain and bellyache if they are not breezing through, using no strategy at all, no teamwork, doing whatever they want, and still have a good chance at winning at the end.
Many of the expansions have been made to be adaptable, some people do not like to be stressed out and feel they "have" to do something, so the game has been made in an attemt to give people a game that can be played and have fun by everyone, whether you like it easy or hard, there are people who want to spend their whole game browsing through location cards and shopping rather than using strategy and teamwork.
The problem with Dunwich is it was the first expansion that came out, when there was alot of fine tuning and working out the bugs. Most of the "difficulty" of that expansion was completely random and relied upon drawing the mythos cards that came from that expansion, and as you already pointed out, if you are adding kingsport cards to the deck, then it dilutes those few dunwich cards down even further, drastically increasing the chance of making dunwich completely useless, with there being no point to go to the dunwich board.
This, however, is a problem that plagues many Arkham Horror expansions, not just Dunwich. Try mixing in 4 expansions at once into the mythos deck, and see if the king in yellow horrifies you that much. See how often any one "horror" theme comes through. It would be like taking friday the thirteenth, nightmare on elm street, aliens, and the reanimator and mixing them all into one single movie that is 2 hours long. The resulting mess is probably going to be partial movie clips from all 4 movies, and wont make sense at all.
Now to the problem. The mechanics in kingsport dont rely on the kingsport mythos cards to spawn the rifts, any card can do that if it matches the pattern at the movement section of the mythos card. But as you correctly pointed out, while every card usually places a rift token, those Dunwich cards are rarely drawn, making Dunwich much less important, to the point of being forgettable.
Does this mean that the kingsport expansion will make Dunwich useless if it is bought? That depends on your group and whether you fall into the "easy peasy" category, or the "we want it hard" category.
Obvoiusly, the problem you stated means you didnt really feel all that "horrified" by Dunwich when kingsport was added, because of the above problems, and felt no need to go there, making it feel like "wow, we actually got a monster into a vortex this time Jim! What are the odds we go another 3 or 4 games before getting another?"
What we do is we make 2 different Mythos decks. We leave all the Mythos cards from Dunwich in one pile, and all the other Mythos cards in another pile. Then alternate between decks during the mythos phase, so that every other turn you will have effectively drawn one from each. This is not a home brewed method of doing the Mythos cards, it is one of the official ways they do it from Curse of the Dark Pharoh expansion, and it guarentees one card out of every two drawn will be an expansion Mythos card( in this case, Dunwich), and trust me at this point priorities will start getting re-arranged rather quickly. You will no longer see Dunwich as useless, and odds are you will have several investigators scrambling to Dunwich in order to do monster control, since there is no limit to the number of monsters that can be on the Dunwich board.
This will make the game much much harder though, so alternately you could create a separate Mythos deck for each expansion used, and alternate between the 3 or 4 mythos decks as you draw. This would add the Dunwich cards every 3 of 4 turns, and would give Dunwich an uneasy "feeling", rather than saturating investigators with fear of the Dunwich board, and either way should add at least some interaction with the Dunwich components.
This can be done with all the expansions, like the king in yellow, in order to make sure those expansions do not get diluted and you get the proper level on interaction.