I wish Metroid had a similar renaissance

It might yet.

Its a good SRPG.... they poured alot of love & hard work into it, and it shows.

The sales reflect it. People noticed.

 I know but before we get some good spinoff like Metroid Warriors we need good titles in the main series which I don't see in the forseable future

I think that if Nintendo takes advantage of the design of FE to make it a graphical showpiece then yes, I think it could grow some more on the Switch. I don't know how much more though. I think I could see a steady 3 to 3.5 million vs the 2 mil it seems set at, 4 mil being the most possible. All depends on how Nintendo handles it of course. 

Edit: and if they bring in the traversable dungeons and some kind of overworld/openworld exploration to link it togeter, taking inspiraton from Echoes, that would definitely help,

You'd have to be incredibly cynical to believe that we aren't getting a mainline series game announced in the near future. Nintendo has been pretty transparent about what they're doing with Metroid. They want to make 2D Metroid, they want to make 3D Metroid, they want to make a new Prime game that revolves around Sylax. They made Federation Force to set that up.

Retro's not working on a Metroid game because Tanabe doesn't know what they're doing. People are just bent out of shape because Metroid isn't a launch game, which was rediculous to think when we were told explicitely over a year ago that a new Metroid wouldn't happen until well into the Switch's life. What's likely happening is that Tanabe is working with some third party dev to make a new 3D Metroid - probably a Prime reboot. I don't know what could be going on with a Metroid 5, but the common thought is that that's Sakamoto's domain.

Point being there are obvious reasons we haven't seen Metroid. I'm willing to bet we'll see a new core Metroid teased as early as E3, going by Reggie's most recent comment. "Come back to me in a year" reads clear as day.

Props to the team seeing what made Fire Emblem inaccessible and worked to improve it. Some hate but aside from Awakening being a little on the easy side, Fates being a little too expensive for the whole experience and the touching mechanic that caused more drama then it should ever have been credited for the franchise is still in good shape.

Maybe it can grow if they take the Waifu/Husbando mechanics to the next level with HD Rumble...

And it's not like Metroid didn't have one before. 2 games were released in on the same day in 2002 (for NA at least) after about 8 years since the last one (Super Metroid). Then the series had a plethora of games with Zero Mission, Pinball, the rest of the Prime Trilogy, Prime DS, and Other M. Unfortunately, sales declined over the course of the trilogy and Other M's lukewarm reception from fans (despite a daring attempt by Nintendo and Team Ninja) didn't exactly help matters. Federation Force, albeit a spin off, is the first game of the franchise since 2010. With all the teasing and such, I can't imagine Nintendo going through another generation without a mainline Metroid. Just hope it can sell and be great as well.

OP: I think the FE can get bigger but I'm not sure that much bigger. It's a SRPG, not the biggest genre in the industry. But in the end, I'm glad it's succeeding after being this close to death. I'm also glad to really invest myself in this series. There are so many games I want to play within this franchise (I still haven't beaten Awakening yet as I wanted to get all the characters). I think the series' future is bright.

There was nothing inaccessible in Path of Radiance or Radiant Dawn. Fire Emblem was a SRPG, so yes, you had to think a little bit before making a move, nothing special about it. 

I'm glad for them that the series is more successful, but games like Awakening have no interest as SRPGs. They switched their focus on more mainstream features like shipping, breeding, grinding, and it works very well for the sales. But after setting a good level of AI, map design, balance, difficulty, variety of objectives (and even scenario/campaign progression IMO), it's a shame to see that they had to get rid of it in order to become popular. I'm still looking for recent SRPGs worthy of Radiant Dawn :(