Launching in 2022 is not "soon." Also, what makes you think MK9 sales would suffer from MK8, a game that released back in 2014 on the WiiU? MK9 can sell 30+ million on the Switch & can be ported to the next Nintendo console. Even the best case scenario for MK8, where it sells another 20 Million, would still be less money for Nintendo than MK9 selling 25+ Million. As for the example of MK Wii, that's not a good one, considering MK DS already pushed the franchise to new heights, and came before the Wii version.
COVID played a huge role as to why Animal Crossing blew up. Its boost can be seen across the board. For Nintendo, their older titles sold millions in a single quarter & broke tons of hardware records. Sony made about a billion in profit in a single quarter in a non holiday quarter. IIRC even Microsoft saw a lot more engagement in their ecosystem. At one point in time, the PS4/Xbox One were sold out, in a year where their successors are on the way. All of the above happened in non holiday quarters. That certainly isn't typical.
I won't be able to convince you about SM selling an extra 5+ Million on the PS4, or that 120+ Million users won't completely abandon the PS4 because the PS5 is out, so I will say that time will tell who's right. Though to be quite frank, a lot of Nintendo fans didn't expect Spiderman or God of War to hit 20+ Million, yet alone do it in two years time.
They don't need to release MK9, so they won't. That 2014 game is still selling a ton in 2020. Another example is GTA V, Rockstar doesn't need to release GTA VI when GTA V is still selling. Both of these games might release on their successors too. MK8 Omega deluxe for Switch 2 and GTA V super complete edition for PS5/XB Series X.