The topographical map work done by Monolithsoft Tokyo was a key element in Breath of the Wild's success. The XCX team was uniquely talented at the vertical map design and development.
Sure, it was a team effort and they can both be proud of every second because it’s brilliant. Monolith was probably making assets and maps more up their alley than with Skyward Sword so they probably pitched an idea here and there. What I’m saying though is that it’s Nintendo that tells Monolith what to do and what not in the end. If Nintendo said the game’s going to be first-person and only take place in Ganon’s Castle, it would’ve been that and Monolith would’ve helped with that.
It's not likely Monolithsoft would have helped with the development of Ganon's castle or any kind of first person stuff since they aren't unique experts on castle design or FPS.
I am not sure how you think the dev process works. Aonuma is the head designer, but he didn't do ALL of the design work, he assigned other designers to design various components while he and the production staff made sure the elements fit together. There were 24 designers in total, 12 of the designers were from Monolithsoft. The rest of the 100 or so devs from Monolithsoft were artists and engineers. The Tokyo XCX team was an indispensible part of the Breath of the Wild development - not because they are "grunts" but because they have unique skillsets in developing 400 square kilometer open world maps, with a lot of vertical elements, and where the player can go anywhere.
Additionally, Monolithsoft isn't a single studio and team; there are four studios in total, one of them worked on Skyward Sword and Breath of the Wild (Kyoto) and the other worked on the map of Breath of the Wild (Tokyo, XCX team); around 100 people were brought on after XCX finished.
With this upcoming game, it is uncertain which portions of Monolithsoft will be hiring new staff and what they are doing it for.