I edited.

Just a assumption.

ya wouldnt surprise me in the slightest, althought Echoes seems to be getting a worldwide launch in May so perhaps that will become a trend.

it happened recently for Pokemon, before 3DS all mainline titles released in Japan a year earlier than the west but they started doing worldwide launches with X/Y.

I have no big problem if they release on both, but why is Echoes not also on the Switch? It's a new release, nobody would be angry about it. And I foresee no technical problem.

 Nintendo assumes the Switch is momentarily to expensive to replace 3DS. That may be true. But it wouldn't hurt if Echoes would've been on both platforms.

Nintendo always releases games late for their consoles.  Remember Pokemon Mystery Dungeon: Red Rescue Team or Mother 3 for the GBA? What about Pokemon Black and White and Pokemon Black and White 2?  What about Twilight Princess on Gamecube?  Nintendo has always done this.  It is not new or unexpected.  Its not even foolish, considering the game will sell a lot better on 3ds than it will on switch (just due to install base alone).

You would be right if said sequel wasn't a 2018-game and a far one at that, as they hadn't to show anything. Gaiden-remake on the other hand probably comes soon. That is something that matters. Especially as porting to the Switch and release Echoes head by head on 3DS and Switch would've been pretty easy.

I just did some searching through YouTube from the trailers for Fire Emblem Fates before it came out in Japan. 
On the Nintendo Direct for April 1st, 2015, (Good Lord, has it really been nearly 2 years since that trailer?!?),
the Japanese trailer: https://www.youtube.com/watch?v=me-9kWzxKoo  , had already showed that there would be 2 different versions of the game, at that time it was known here as Fire Emblem: If, along with the release date of June 25th, 2015.
Meanwhile the American trailer, https://www.youtube.com/watch?v=94CCxVMltQY , didn't show either of those 2. We wouldn't know about the split versions officially until E3, and at E3 we got a "Coming 2016" release window, while the Japanese release was 2 weeks away at that point. 

So the fact that we got a 2018 release window right off the gate alongside Europe and Japan as well gives me hope that this is releasing simultaneously worldwide, but I won't put too much hope in it.

Itd good for me, i don't plan on getting a switch and more games on the 3DS means my 3DS stays relevant for longer

I don´t see any problem with it. There are a lot of games to be launched for the 3ds this year.
I do think that Nintendo should focus on the Switch right now but it´s just one game. Things will be really different in 2018. I don´t see new 3ds games after 2017