A 3DS successor isn't entirely out of the question, if there is sufficient interest and demand...and there will be. Don't think so? Just remember that there is still a niche market for PS Vita titles in the West, and think about how popular the 3DS is versus the PS Vita. If Nintendo have any business sense whatsoever - which they might; anti-consumer they may be, from a business standpoint a lot of their choices make sense - there will be a 3DS successor. Or, at the very least, a Switch Mini, more on par with the XL handheld sizes and at an affordable price point. That might seem like the more likely option, but then, this is Nintendo. Putting the NES/SNES Classic Mini games on the Switch Virtual Console would have seemed like a more logical move too.
What I'm most salty about is that its unlikely the Switch Pokemon game will come to the New 3DS. The New 3DS is more than capable of improving performance - it ran Hyrule Warriors at a more consistent frame rate than the Wii U. It squeezed in Xenoblade Chornicles 3D with fairly good resolution all things considered with no noticeable slowdown. It has Fire Emblem Warriors. The 3DS might be old, but the New 3DS is a pretty solid improvement. That could have at least three or four more years of life in it, and another generation of Pokemon titles. It should, in fact. There is no excuse for it not to.
I refuse to accept that it couldn't handle a Pokemon game built on the Switch too, and considering the visual upgrade likely will not be that substantial - although this does depend on the time we wait for release I would imagine - there is literally no excuse for them not to put it onto the New 3DS. Even the install base argument won't work, because it's been a while now since you could actually buy the old models, and even the standard sized 3DS has been phased out. It's industry bullshit, plain and simple.