I could see Nintendo doing a project in collaboration with From Software but they'd never buy them out. The problem with takeovers like that is that you never know what the aftermath is. We've seen many cases (including Rare) where a company buys a studio they've never worked with before and in result many of the key staff members jump the ship. If the staff doesn't like to work exclusively for Nintendo consoles, there's no advantage for Nintendo to buy them.
That's why Nintendo only acquires studios they've been working with for a long time (Retro, Monolith, etc.). They know the studio structure won't change dramatically after the acquisition. If they want to have a studio to make new kind of games (like FPS games or something), they should just form a new studio for that or do it in collaboration with someone.