Announcements of new systems don't ever have any visible effect on sales of systems they're replacing. However, the actual release of new systems does have an effect that is almost always immediate and pronounced. DS sales were already declining before the 3DS came out, but started to absolutely crater once the 3DS was released. The DS wasn't doing amazing when it first came out, but even then it caused GBA sales to have a noticeably sharp dive. We see the same pattern in other product lines besides Nintendo handhelds. Perhaps the only time a new system didn't have an immediate effect on its predecessor's sales was the PS3 in the U.S., and that might have had something to do with the $500 price tag at launch. It wasn't until the PS3 was reduced to $400 that the PS2's decline in sales started to experience a pronounced acceleration. But that was one exception for a rule that seems to be true in every other case.
Since the Switch doubles as both a home console and a handheld, and will likely be marketed as such (the reveal video even focused more on its portability than the fact that it's also playable at home on your TV), I expect it to cause an immediate and pronounced YoY drop in 3DS sales. The 3DS is doing fine now, but it could very well be a last gasp. The 3DS will almost certainly be down quite a bit in 2017, by holiday 2018 it will likely be dead in the water sales-wise, and it could be discontinued before the end of 2019.
Exactly, however I wouldn't call the drop of gba sharp though because it was already falling, maybe ds release accelerated the decline.
DS lite killed gba in 2006, fat ds only sold 18-19m between 2004-2006, and that's why I'm always saying that ds didn't cut gba's life in 2004...
As for the announcements...
Excuse me shadow, but Nintendo announcing the release date of NX back in March, affected severely wii u sales later , since people had NX on their minds as the wii u successor.
Wii u was slightly yoy down in Jan-Feb-Mar, but after the announcement, it started having big yoy/mom drops.... That's not coincidence!