It goes both ways, data shows that Switch isnt hurting PS4/XBO but it also shows the success of PS4/XBO and the lack of most AAA multiplats is not preventing Switch from doing well.
By the time PS5/XB4 arrive in 2020/2021, Switch will start to reach impulse buy territory with a price cut or two (sub-$199), some revisions that cater to specific audiences (Mini, TV, Pro) and have built up a large existing library (possible Nintendo Selects line) so it will easily be able to coexist with PS5/XB4 for a few years.
It's not preventing, so far.
If switch's market keeps beeing PS4 and XB1 users, then that market will dry up sooner rather than later, because it isn't renewing itself - again, so far.
2020/2021 will be a completely different story.
The 199$ price tag, or lower, doesn't carry the same wait it did before; When consumers adopted PS4 and XB1 with a price tag starting at 400$, the way they did (surprising even developers), that showed that consumers don't see price as a big factor anymore. As long as marketing works and promises are made, they will pay the price - like others do for Apple products, for example.
Switch needs something more than rely on a concept and 1st party titles alone.
The console had an amazing 1st year, but if Nintendo wants to really do well and really change things structuraly, that alone won't cut it.
Personally, the only real structural changes i have seen was the change in target audience and Nintendo betting on online gaming, like never before.
If Nintendo will be able to coexist with PS5 and XB4 will probably be more because of those two consoles and what they will bring, than Nintendo's actions. But that's me.
As zorg has pointed out, the sales tend to indicate that Switch is not directly competing with PS4 and Xbox One, and therefore wouldn't compete directly with PS5 and Nextbox either. Furthermore, Switch sales aren't being driven by AAA third party ports and it's barely getting any as it is, so it won't matter much if those stop.
But they are competing! They just arrived at a time where there were enough PS4 and Xb1 users looking for something new.
Not to forget that Switch had a once in a lifetime launch: concept, return to roots Mario and Zelda, MK8 and Splatoon. All in the same year.
Nintendo has yet to show that they can remain strong in the years to come.
To me, one or two system sellers (Pokémon Go for Switch is still a maybe) won't be enough. Specially if they are both released towards the end of the year.
Two years from now we will know for sure how much Switch's library has increased in the system seller department. Then, we will see if Switch can fight not one. but two new consoles!
Agree, not just that when PS5/XB2 arives they will have $400+ price points while Switch will probably have around $200 price point with multiply revisions present on market, but Switch main point and biggest advantage over competition will still be present and that's of course full handheld play.
First, the hybrid concept is selling but like any other system seller aspect of a console, it loses selling power over time. Even if just because there are two new consoles.
Second, as i wrote above, consumers didn't care that PS4 was 400$, that XB1 was more expensive (had to do more with it being bundled with something that gamers didn't care and had no alternative, plus other things).
If PS5 and XB4 is everything gamers wanted from next gen, you can bet a 200 Switch won't have the effect you think it will, specially when Switch isn't stealing costumers from MS and Sony.