The thing with Banjo and Steve that secures them for me is they just kind of feel like the most sensible (DLC) third party characters to prioritize, for opposite reasons yet fairly equivalent.
Banjo is for the fans, Steve is for the masses. Much like Mega Man before them, I anticipate Banjo & Kazooie are posed to be a series staple who their parent company will let Nintendo do just about anything with. Much like Sonic and Pac-Man, Steve is a universal icon whose very presence is a selling point for fans who only recognize a handful of characters.
I try to be fairly conservative about guest fighters. Full transparency, I would be much less upset to lose most of them than most people. But coming off the heels of Ultimate, most of the main "safe" third parties are agreed to be... the ones we already had in Smash 4, which I think is unrealistic. We simply aren't going to lose all of these highly anticipated, extremely popular newcomers. We will probably lose some, maybe more than some people would like, but I think we will maintain roughly half of the third parties we gained by proxy. Many of them are very popular, are from companies in good standing with Nintendo, and would be annoying to buy as DLC twice in a row.
I can't really foresee an outcome where Banjo and Steve are low priority. Even under the circumstance where Microsoft plays hardball for base roster, much like I see happening with Disney and Sora, the worst it will do is delay these characters from returning until DLC. And I'm not really the kind of person who feels comfortable flooding the roster with a dozen third parties... I just recognize how much series buzz rests within these characters, and Microsoft is now one of Nintendo's most valuable partners who is certainly going to want to come along for the ride once again in some capacity. There's no reason not to.