I think that it's certainly in its last year of major support but a mainline Pokemon game and the NX not even being out yet mean while it's "on its way out", its not nearly as close to its eminent end as you are seeing it to be. More worth it for TPC by far.
But if Sun/Moon are Gen 7 then it's not just 'a mainline Pokemon game' that's a commitment of 3 years to a generation. Whether they pull a Gen 5 and don't give us DP remakes to accompany them, and instead another supplementary game, we're going to be in this weird situation of Gen 7 releasing games on a system that is already quite old in 2016, let alone 2017/18.
You can excuse it with Gen 5 for a couple reasons;
1. The DS was a huuuuuge success, Nintendo's most successful handheld ever. It had a long lifespan
2. Black and White came out 6 months before the 3DS in Japan, and a few weeks elsewhere.
3. We only had one pair of games that released on 'old hardware' with B2/W2 releasing Summer 2012 on the DS in Japan, around 18 months after the debut of the 3DS.
Let's just assume that a form of the NX is releasing this holiday (Nintendo is crazy if it isn't). This would put a timeline on games releasing something like this-
Fall 2016 - Sun and Moon release on the 3DS, the same time as the NX (maybe they have some exclusive features but all in all they are 3DS games). This means they are advertising games for an old platform the same time as a new one.
Fall 2017 - We maybe get DP remakes or some kind of supplementary game be it sequels or a third version
2018 - If 2017 wasn't a gap year then this is where they might take a break with a release of a game. That's only if we don't get 5 games - Sun/Moon, DP remakes and a third version
2019 - Seeing as Gens last 3 years minimum this would be the earliest we could expect a next gen Pokemon game. Are Nintendo really going to sit on this for that long, when they could have released a third version (assuming S/M aren't sixth gen) this year? Are they really going to want to keep the DS brand around that long when their naming conventions have caused them nothing but trouble this gen?
A lot of fuss over franchise I know, but this is Pokemon, their second biggest seller after Mario, this stuff matters to them. Pokemon moves consoles, and if they want NX to be a success then they need Pokemon on there quick.
EDIT - One other thing I forgot to mention is that the 3DS is their worst selling handheld, why on earth put 2 gens on that? GBA did way better than the 3DS and that only had one pair of new games, remakes and third version.