If Nintendo bought Sega, someone would have to make the games that use Sega's IP. So who makes Sonic for Nintendo, then? The same dolts working on it now? It's not like Sega currently is very talented. They USED to be good and have some great historical IP but someone has to work on those games or there's no point, at least from our perspective (ie: Nintendo might like selling crap Sonic games to suckers). These games don't make themselves. Either Nintendo uses the existing Sega staff, hires more staff, or moves their staff to Sega IP (at the expense of Nintendo IP).
And even if it worked out and Nintendo had Sega back on track, it won't feel like Sega. The similarities between Nintendo and Sega was simply that they were the top videogame companies in the world for a while and they made really good games. But I could never see Nintendo making Sonic, Streets of Rage, Panzer Dragoon, etc. It's not Nintendo's style. It would be better for gaming if Sega just didn't suck anymore. I want other developers to make games as good as Nintendo but I don't want them to make games LIKE Nintendo's. I have Nintendo for that. Sega was probably never going to make Pokemon and Nintendo would never make Shenmue. That's good. The artform needs that kind of variety. I figure if Nintendo bought Sega, Sega's new games would just feel like Nintendo games. I would assume they would be good games but it would lack the Sega identity that made them matter in the first place.
It's like how Square Enix didn't create some super RPG company. All it did was take away two different approaches to the genre and replace it with one. You don't want Nintendo Sega. You want Nintendo AND Sega, two different companies with different approaches to videogame design both making great games. The constant mergers and acquisitions in this industry aren't something we should want as it will just result in safer homogenus game design.