Downward sprial? USUM sold better than Crystal and Emerald, it'll pass Platinum soon, and it's currently only 650k behind B2/W2, games that had actual effort put into them. USUM was incredibly lazy and still put up numbers on par with the previous third versions and sequels. Not to mention, SuMo released after Pokemon Go and managed to outsell R/S and B/W, both of which launched on larger userbases (significantly larger in B/W's case). Even though Diamond and Pearl were on a console that sold 80 million units more than 3DS, the games only managed to sell 1.38 million units more than X/Y, and 1.57 million more than SuMo. The core series has not seen any decline, and in addition, Pokemon Go has multiplied the Pokemon Company's annual profits by a factor of 8 compared to the previous decade. As a franchise, Pokemon is in a stronger position right now than it has been in at any point since 2002.

USUM are two games. 

Sun and Moon failed to sell more than X&Y I don’t need to say anything more.

The games have been STAGNANT for a while now despite the desperate efforts of Gamefreak trying to appeal to young gamers each and every time. User base are exactly the same leaning even older with Sun and Moon.

Instead of trying to appeal to the demographic they have. They tried many many times to the younger markets. Sooner or later people will get tired of that. 

Its already happening. Forums, youtubers esp of Pokémon content are falling out. (Trust me I used to be on Pokémon forums. They are mostly dead and smogon isn’t the same at all.) YouTube people I used to follow completely stopped making Pokémon content or just play only old Pokémon games or moved on to smash. 

Nostaglia leaning, causal grabbing, same old formula will be the death of the core franchise. 

So S&M selling less than XY is a sign of decline? How close are they in terms of sales again?

And Pokemon Let’s Go is mostly aimed towards Go fans, who are more likely to be young adults. Some of those players haven’t played Pokémon since Red/Blue or maybe Gold/Silver. 

And as for YouTubers, they just move on to other stuff. It’s natural. Unless you’re a dedicated Pokémon player who only plays Pokémon, you’re not gonna focus on the same franchise forever. It doesn’t mean they no longer love Pokémon or don’t find interest in playing them anymore. Your examples may be as anecdotal as when others have said their co-workers who have played Go are interested in getting Pokémon Let’s Go.

USUM are two games. 

Sun and Moon failed to sell more than X&Y I don’t need to say anything more.

The games have been STAGNANT for a while now despite the desperate efforts of Gamefreak trying to appeal to young gamers each and every time. User base are exactly the same leaning even older with Sun and Moon.

Instead of trying to appeal to the demographic they have. They tried many many times to the younger markets. Sooner or later people will get tired of that. 

Its already happening. Forums, youtubers esp of Pokémon content are falling out. (Trust me I used to be on Pokémon forums. They are mostly dead and smogon isn’t the same at all.) YouTube people I used to follow completely stopped making Pokémon content or just play only old Pokémon games or moved on to smash. 

Nostaglia leaning, causal grabbing, same old formula will be the death of the core franchise.