So long as Teshub and blob are still around, Brock and Dawn will never be forgotten.
I'm sure they will be if Brock and Dawn appear in the same cameo episode. I never saw Ash, Brock and Dawn as a "set". They were a trio (and I feel that they were a better group than Ash, Brock, May and Max), but I don't consider Brock a part of any "set" other than the Kanto trio.
To me, Sinnoh was Ash's adventure with Dawn, along with a creepy, perverted Asian man who followed them around and cooked food for them.
I suppose Ash and Dawn will be remembered because the two were very much alike...? It felt less like Ash was a mentor (Ash and May) and more like Ash was simply Dawn's equal, who happened to know more about the Pokemon world than Dawn. I liked that.
I personally hope Iris and Dento leave after Isshu (Dento moreso than Iris) simply because then we won't be incredibly sick of them by the time they finally do leave. That and we'll remember Ash, Iris and Dento as a "trio" on it's own, and not a couple travelers with a new member to replace an old one (although I feel like Iris's starter not being an 'Isshu Starter' is kind of setting up for her continuing on into Gen 6 with Ash). Then they can be remembered simply because they were there for Isshu and Isshu only.