(Everything I say will have probably already been stated somewhere in the thread, I haven't read it all)
It is 100% fair, in my opinion, to move onto the 3DS. The DS has had a fantastic run, but as soon as the 3DS was announced 2 years ago now (yes, it's been that long!) you had to see this coming; the end of the DS' life cycle. Pokémon has had two full generations on the DS, it's time to move on. If you're wanting to continue handheld gaming with Nintendo then a 3DS purchase is kind of inevitable.
As for the price "issue" well... I don't really think it's an issue at all. They're not as expensive as they used to be (back then they were, like, £220 or equivalent upon release!), and you have until October to start saving some pennies up to get one. I know it's easier for some people than it is for others, but yeah. 10 months (or just under) is quite a long time. And, I know many people will want the games as soon as they come out, but there's always Christmas. Again, I know not everyone does Christmas, and maybe the 3DS is more than you'd usually get for Christmas for some of you, but still. There are options. There's pre-owned as well. You can get pretty good deals on pre-owned consoles if you look in the right places.
As many people have said, Pokémon being on the 3DS will probably increase 3DS sales, rather than Pokémon losing sales because it's on the 3DS.