A game there's even less demand for.

I disagree. Don't even know how you conclude that from DKTF's sales vs MP3. 

There is an outcry for an HD metroid prime. It would at least surely move more Wii U's.

Because sales are the only real measure of how big a fanbase is. Metroid's is louder but smaller.

You have to note one thing,singleplayer based games tends to sell lesser than multiplayer based games

I still think that MP4 would sell more on hype alone. Its obvious DKC was a game that was wished for, but TF wasnt. It was too soon for another DKC, but not for another metroid prime.

Metroid Prime 3 sold a few 100k more with an extra 90 million install base. Based on the love it gets on the internet, u would think Metroid was a huge series but it's very niche.

It sold 600k more, and MP1 & 2 also sold more. Userbase doesn't matter for the Metroid franchise.

To be fair, I'd estimate that a little less than half of that 100m install base on the Wii would be what you'd call 'core gamers' based on the sales numbers for some of Nintendo's significant non-casual games (Animal Crossing, Smash, 3D Mario and mainline Zelda for example) compared to similar series entries on the Gamecube.  The only games I can remember off-hand that got significant boosts becuase of the casual crowd (besides casual games) were 2D platformers, Mario Kart and party games.

This site has Prime 2 & 3 overtracked. Someone with official data made a thread on Neogaf a while back which had a list of Nintendo-published games that crossed 1 million.

Prime 2-1.10 million

Prime 3-1.41 million

Tropical Freeze-1.12 million

Those Tropical Freeze numbers are from Dec 31, 2014 so that number has grown since then, it's already ahead of Prime 2 and may even surpass Prime 3.

Regardless, u can look at Metroid II, Super Metroid, Fusion, Zero Mission, Hunters, Other M. With the exception of the original & the first Prime, Tropical Freeze will go on to sell similar numbers to the majority of Metroid titles.