With this Nintendo should be able to comfortably get the 3DS past the 70m mark within this fiscal year. Good announcement, because the only 2DS version that has been available before isn't exactly pretty and does not have a clamshell design.
Skimming a bit through this thread suggests that there are people who believe that this announcement could cut into Switch sales. That's not how it works though. This is going to add to the total of the 3DS family without taking anything away from Switch, so it can only be viewed as a positive thing.
Ya the person who is getting their first 3DS family device in 2017 very likely does not belong in the same demographic as someone looking to get a Switch in 2017.
They can easily coexist 1-1.5 years without one hurting the other.
When the herd loses its way, the shepard must kill the bull that leads them astray.