I wouldn't be surprised if Nintendo shut Retro down. That studio has been sleeping in its laurels since 2014.

Nintendo can shut them down.
It doesn't matter how good the development team is, if Nintendo doesn't want it, or sees no value in keeping it afloat, they can just shut Retro Studios down.

It doesn't matter if Donkey Kong Country: Tropical Freeze was great, if Retro Studios cannot deliver the games Nintendo wants them to make, they are useless to Nintendo.
Retro Studios aren't independent, they don't get to decide their fate.

Nintendo can hire other studios to make Donkey Kong Country titles, if they want to.

Just looked up Naughty Dog and Santa Monica Studios to compare. Retros is low, but it actually has hardly any reviews compared to other companies.

I still want to believe they're working on a brand new IP that will be seen as a piece of videogame history 20 years from now. They just gave me to much fun and quality with the Metroid Prime Trilogy and the DKC games for me to believe these rumors.

If you'd have read developer reviews from Retro Studios in the run-up to the original Metroid Prime, I imagine people would have said a similar thing - Retro was in absolute chaos until Nintendo took closer control.

It doesn't take a lot, at this point, to guess that at least one Retro project in the last four years or so has entered development hell and that would understandably put a lot of pressure on the staff. When the Star Fox Grand Prix rumours broke, Kotaku were reporting that another Retro title was in development hell and likely on track to be cancelled, which might be behind these negative reviews. As for Nintendo shuttering the entire studio, if the Star Fox game is real, I suspect that Nintendo will try to get that out the door first. Like I said, back in Retro's early days, the studio was absolutely chaotic - and Nintendo were throwing money at them, burning cash. Despite the losses and the lack of results, Nintendo still hunkered down, took control of the situation and kept faith with the staff they kept on at Retro, and the Prime games were the result.

Not saying we should expect history to repeat itself, mind, but I don't think this should be taken as the studio's death knell, either.

EDIT: Should also be pointed out that, while Retro haven't put out a new game, they did handle the Switch port of Tropical Freeze - so they clearly still are active to some degree.

This is concerning, Retro should have been one of Nintendo's top studios

Retro seem to have lost their touch, a la Rare after the Microsoft buyout.

Nintendo should really invest in some new (preferably western) studios. They don't have great third party support, thus most of the software output has to come from first and second parties.

They can.... but whos to say the quality in games would stay as high?
Would you want some random studio makeing DKC games?

A number of studios right now have negative reviews it doesn't really tell us anything especially with only 5 reviews from 8 years.

Considering Nintendo has not yet killed them, apparently not.