Misty is not going to be forgotten, we still see Tracey from time to time, right? The point is before she was just left out of the AG series, but now that she's left out of D/P too, that pretty much cements the route the writers are taking with the series. Dawn will probably get at least 4 years on the show like May did, and if she leaves too, the writers are more likely just to introduce yet another new girl onto the show. The point is while Misty was popular, both girls who came after her, (May and Dawn) also became widly popular characters in the fandom. So it seems that even with Misty gone, the writers found success with the two girls who came after her anyway.
Misty is a well-liked character but once the Johto saga started it became pretty obvious that the writers had trouble figuring out which direction to take her character. For the majority of Johto, Misty isn't focused on at all and just left on the sidelines, and her goal of being a Water Pokemon Master is touched upon only once with the Whirl Cup and that's it. After that ends she doesn't do much for the rest of Johto, and after that...well that's it.
If you ever rewatch the entire Johto saga in retrospect, it becomes pretty easy to see why that saga turned out to be her final region. It seemed like the writers didn't have a good grasp of what direction to take her in, and her character suffered because of it. The reason why she got those special episodes and appearances in Hoenn was so the writers could tie up some loose ends with her character now that they knew what to do with her. Its pretty obvious when you see the very first thing they do with Misty in AG is finally evolve her Togepi and have it leave. Realistically that should have happened back in Johto, but the writers didn't have any idea what they were doing back then.
I'm sure we'll see Misty show up for a few eps in D/P at some point, but if the past is any indication it won't happen that often. I understand that Misty fans are disappointed that the writers never used Misty to her full potential, but there's nothing that can be done about it, so just accept whatever appearances she gets between now and the end of the series and take it as a bonus.