Compared to recent years I feel like Pokémon has been less popular than it used to be. Sure Pokémon is not as big as it was in Gen 1 and Gen 2 to an extent which is understandable. But compared to Gen 4 and 5 it seems like Pokémon is less successful currently. X & Y are currently the least most successful set of games (not counting remakes) with around 14 million sold however that's still reasonable and close to Diamond & Pearl which have sold over 18 million. The 3DS has the lowest sales of any Nintendo handheld to date despite selling reasonably well and I've seen many people buy a 3DS mainly for Pokémon. If Nintendo handheld sales continue decline it's possible Pokémon will slowly go down with it unless Game Freak decides to release the main series games for smartphones which probably won't happen but is possible considering Game Freak has put games on non Nintendo systems. However that would also cause Nintendo handhelds to lose lots of sales.
I'm sure Yo-Kai Watch stole some thunder from Pokémon especially in Japan however Yo-Kai Watch seems to be on the decline after 2014 and of course Yo-Kai Watch isn't doing too great in the west compared to Pokémon. But the main factor is smartphone gaming. People of all ages have shifted to phones and a lot of kids are playing smartphone games over 3DS. And while the target audience has been kids I see more teens/young adults who are into Pokémon more than kids (at least when it comes to the video games).
When Diamond and Pearl came out lots of kids got them. That's when smartphones weren't around yet. However when HeartGold & SoulSilver came out the kids who got them were slightly older and a lot of teens and young adults got them too but I think several people who bought them grew up with Gold & Silver. When Black & White came out, it seemed like more teens and adults bought them rather than kids. I think by HGSS former fans began to get nostalgic over Pokémon and get back into it. And I'm very sure XY and ORAS were bought by tons of adults too. I felt like I was one of the many people who were around 17 nostalgic over Gen 3 and bought Alpha Sapphire on Day One at the same Gamestop I got the original Sapphire.
So I feel the Pokémon series is going to become unhealthy if not enough kids play the games and way more adults play them. What might help the series is to attract new younger kids but considering most kids play on smartphones it's going to be harder to do. Gen 6 has been so streamlined that it could be easy for more kids to get into the games and while there are always newcomers to the games who are kids, there isn't a huge amount. Not to mention I've seen some older fans complain that the series hasn't evolved enough and has become too easy but I felt like Gen 5 and 6 were some fantastic directions for the series. Also the easy difficulty of Gen 6 did not bother me much.
Maybe there are tons of kids who like Pokémon but don't play the video games. There are so many other media that Pokémon has been spread to that it doesn't feel like just a video game series. The anime love it or hate it, is more widespread worldwide than the video games. I mean the anime has been dubbed into like 40-50 languages or more while the games were only translated into 9 so far. And it's possible that there are still many kids who watch the anime whether its on TV, online or whatever and some kids get their Pokémon fix that way.
Also there is the Trading Card Game which is like battling Pokémon but in card game form. So maybe there are some people who play the TCG and don't play the 3DS games and get their Pokémon fix that way. Because I think the TCG can be just as fun as the games. Some people might just collect the cards while others play them and go to tournaments. So even if the games decline the Pokémon TCG might still stay relevant considering there isn't much competition other than Magic or Yu-Gi-Oh. I also think the TCG Online has been was of the best things to ever happen to the TCG because you can just play the TCG online and make collections and it's easier to find people to play with. And I heard there were record sales for the Pokémon TCG in 2015 so I think the TCG will be fine.
Also there have been Pokémon apps coming to smartphones including Pokémon Go which I don't care for at the moment but that might seem like a great idea and will be healthy for the franchise especially among people who don't play the main games.
Also I still think Pokémon toys still do pretty well compared to other toylines even if toys in general became less successful with the popularity of mobile apps. I feel like Pokémon merchandise in general has been better marketed and presented since Gen 6 came out especially when the Pokémon Center Store launched online.
There are even some hipsters (no offense) who are really nostalgic over Gen 1, 2, or 3 and will still play those games on their Gameboys or illegally on their phones, and do anything relating to those generations.
So I mean Pokémon is still big in some way but in terms of the main series games, are they going to continue to decline? Will Sun & Moon be even less successful than X & Y? Will Pokémon still remain popular even if the sales of the main series games decline? I'm just looking for an answer. Maybe in 5-10 years the people who grew up with Pokémon will introduce it to their kids and might create new fans that will get into the series as well.