I keep seeing people say that Odyssey overshadowed FE Warriors...
But not enough people saying that they put the nail in the coffin with the announcement of the Hyrule Warriors port...
I imagine a lot of people who might have been planning to get FE Warriors down the road will be more likely to get Hyrule Warriors now...
Though if there's any silver lining to this, it might result in a price drop for FE Warriors...

The Problem with FE: Shadows of Velentia are as follows:

1) It's the third FE game "generation" on the 3ds, and Conquest wasn't out that long before this. I'm sure people would have prefered they made and Advanced Wars game on the 3ds rather than a third FE game.

2) I may be wrong on this, but isn't it a remake? If it is a remake, those hardly ever sell well, and if not then I wonder who else is like me and skipped it due to thinking it was a remake? Metroid Samus Returns, I doubt, sold as well as Nintendo would have liked and absolutely would have sold better had it been a new Metroid 2d (A Link Between Worlds wouldn't have sold nearly as well if it was just a remake of a Link to the Past).

3) Age of the 3ds and the hype being with the Switch.

According to VGC Echoes sits at 600k, Awakening and Fates are at 2 million. That's solid considering the 3DS is in it's twilight, Echoes released only a year after Fates in the West, Fire Emblem Switch was announced before it launched, Echoes is a remake of a very unconventional game, and while good the game isn't that good. To sell as well as Awakening/Fates was never realistic, though I did think it could pull a million, but eh.

I highly doubt a Switch version of Echoes and Fire Emblem Switch releasing in 2018 could exist in the same timeline. I'm certain Echoes happened because it was easy to make happen, all the engines n assets were already there from Awakening n Fates. To make a Switch version would require more effort, which could impact the progress of Fire Emblem Switch. I know you said a straight non-HD port, but I don't think Nintendo would ever do that for a retail release, gotta keep up that quality image!

As for Fire Emblem Warriors, both versions combined sit at 400k, maybe it could creak past 500k lifetime. For a spin-off of a series that sells 2 million, that's not bad. Even with a better roster I don't think you could expect much more.