I dunno, they've pumped out XB 2, Torna and the remaster in only 4 years, plus X for Wii U and the original in the Switch's later years. Thats three games, a remake and an expansion in ten years, there aren't many other Nintendo franchises that can claim that. Plus the team has been helping out on other things. That might not be Rare's N64 output but no one puts out games at that pace anymore, its just not possible.
Nintendo actually does a lot of third party collabs, right now they have projects/series going on with Platinum (Bayonetta), synSophia (Style Savvy), Jupiter (Picross), Kuju (Art Academy), Koei Tecmo (Warriors), Sega (those shitty Olympics games, which Sega self-publish but Nintendo gets a cut from), Arika (Dr. Mario), Spike Chunsoft (Mystery Dungeon), Square (various collabs and IP shares), alphadream (now bankrupt), Bandai Namco (Snap currently, but also lots of other stuff), Ubisoft (same as Bandai) and Genius Sonority (extremely shit Pokemon spinoffs). They also have groups which are technically third party, even if they only work on Nintendo projects, like HAL, Next Level, Intelligent Systems, Camelot, Grezzo, Sora, Gamefreak, Creatures, Ambrella, etc.
Its a deliberate choice by Nintendo not to absorb those companies, though it looks like they've made an effort to at least get them all as physically close to Nintendo HQ in Kyoto as possible. I'm not sure if that is for liability issues (have fun with your bankruptcy, alphadream, thanks for taking on all of the risk with those games,) or to keep quality higher or what but Nintendo is generally speaking the size it wants to be.
Ugh, its been four months.... time to update my big autism chart on which publishers own which studios and what they are working on.