So Nintendo shouldn't try to expand that fanbase for the next and future Metroid games? Only cater to the already existing fans?

They certainly should. They just shouldn't dump a ton of money on it.

So your argument is it was a plus because the game was rubbish? Not much of an argument for MP especially as the MP for the game was mediocre.

Splatoon's single player is pretty much a tutorial for learning the game much like many other online shooter what makes the package appealing is the unique gameplay and constant free DLC.

Or Prime 4 with MP will still bring the same single players who may or may not dabble in the MP while multiplayer gamers will still stick to COD and the like what you're trying to pass as a fact isn't one it's what you hope could happen you're passing of one of many outcomes as a forgone conclusion.

Rockstar Games dumped a ton of money on Red Dead Redemption and sales have gone up from 2 million units (Read Dead Revolver) to 13 million units (plus digital sales).

How much of that would you say is more from the success of GTA San Andreas and IV rather than the money dumped into RDR?

What about the success of Breath of the Wild and Super Mario Odyssey? Haven't they built up trust that Nintendo got better in bigger games? Shouldn't that confidence rub off to other Nintendo IPs?

Sure it should. If they were to make an action/adventure 3rd person Metroid in a similar vein to BotW/Odyssey.

I haven't even looked into Metroid in ages, but wasn't Prime typically a FPS/platformer thing?

No, you still dont get it, without online would be even less appealing than it was. But again, we talking handheld game and online on DS, so online mod definitely wanst best experience and in that sense we would have huge difrence with potential online mod on MP4.

Tutorial can be long 5-10 minutes, there was no need to made hole  5+ hours length single player campaign. Also not just that Splatoon 2 also has single player campaign, but DLC will add additional single player campaign, so its obvous we dont talk just about tutorial.

No, this is fact, MP4 without online will bring only single players, but MP4 with online will bring same single players but could bring also some players that have intrest in online also or just in online. Again you have Splatoon like example just with reverse situation, online focused game with single player campaign. You still have hard time to realise that online mode can be only plus for game, not minus.

If "I" were Nintendo (as a fan), I'd pour everything into MP4.

If I were heading Nintendo as a businessperson, I would play things somewhat safe with a budget under 40M. I would make sure that the game was large and well polished, but not sell the farm over it.

...but I want it to be a huge AAA that re-invigorates the franchise.

Again your argument is MP was appealing because the game was poor.

You're not making much sense in what you're saying here tbh all I can say is single player is short and just a way to walk players through using different weapons and specials so is essentially an extended tutorial much like the upcoming DLC will be to introduce the Octolings as playable hence why its a short single player campaign.

You mistook your own assumption as fact hence why you've now changed your argument to could now and you're having a hard time understanding that not all games are geared for multiplayer and can be much better off with out it I even gave you examples.