it's not even confirmed if Nintendo would publish SMT5 (and they haven't published an SMT game before, nor did Atlus need help in doing so). and Atlus sure as hell won't let anyone else touch Persona
Every 3rdParty exclusive (Octopath, NMH, SMT V etc. ) announced during the early 2017 Switch press conference so far was published by Nintendo in the west. NoA was also the one to confirm a western release of SMT V last year - ages before we usually would get a confirmation for a likely 2020 game.
SMT IV also was supposed to be published by NoE and they had the cross promo for FE:A and SMT IV int the states. So they already had similar plans on 3DS that fell flat because the game wouldnt have sold enough to justify the localisation costs in non-englisch languages.
The likelihood of Nintendo not publishing the game in the west is seem low to me. I doubt Atlus/Sega would decline that offer since that deal is probably one of the the main reasons why Atlus felt safe confirming the project as an Switch exclusive this early.
Persona is something they can and likely want to handle on their own, it will likely be multiplattform and is their established big IP. With SMT V they will try to make the next big step to an even larger audience with Nintendos support.
Same way SE will gladly take Nintendos help for Dragon Quest, The World Ends With You and co - but will handle the multiplattform FF games on their own.