Why not see it as potentially positive? Since how long we all rant about Nintendo is being old and stubborn, is not innovating structurally, is not moving forward fast enough with certain technologies and businesses (online service, smart phone business, anyone?)? Such an investor could really bring in new ideas/moving things forward. I personally think that Mr. Furukawa is very smart, he's of a younger generation and he knows all the (old) shortcomings of Nintendo. I truly think he wants that Nintendo becomes an overall better company, a newer company (but not a hipster company). He knows that changes can only come if he listen to outsiders that's why is willing to listen to ValueAct. I think it's smart.
Personally, I wish Nintendo would start an adult brand (of course they have to name it differently) for games with adult content. I know, Nintendo doesn't like blood, violence and war themes but there are so much more adult themes. I also know that Nintendo's current in-house developers neither have the desire nor the skill to make such games with great quality but Nintendo has so much money, they should buy studios or build up new ones from the ground up for the development of such adult games. For me it's just silly to have a situation that if you are an adult gamer you have no choice than to also buy a PlayStation, Xbox or a gaming PC. As much as you love Nintendo, certain content you can't have on it (or just in limited quantity and quality). Nintendo always comes with the excuse that their games appeal to everyone including (young) adults. Yes, Mario Kart, Smash and even Mario Party are great party games where even (young) adults can have great fun with friends but ultimately, Nintendo says to such people: "Thank you for buying Nintendo products but for you to become fully satisfied, we are sorry, but you have to buy a either a PlayStation, Xbox or gaming PC."
You're parroting arguments that I haven't heard since the Wii days. People these days don't even see Nintendo games as "childish" anymore. Literally every other major publisher is already making "adult" games, and doing a fine job. Nintendo should just keep doing their own thing; there's clearly a demand for it, and they're the only ones consistantly pulling it off. Each platform has it's own pros and cons. It's just as silly to complain about having to buy another system for "adult" experiences as it is to complain about having to buy a Nintendo system for the unique experiences that they offer.