No way in heck is Nintendo going to go that long without a brand new Mario Kart for the Switch. It's one of their premier franchises and a huge money-maker. 8 Deluxe functioned great as a launch-window enhanced port of a game that didn't quite get the recognition it deserved being a Wii U game, but considering it was originally released in 2014, you can guarantee there's work being done a new game by now.
And DLC for Deluxe doesn't really make sense considering it was supposed to be the "definitive" version of 8 that already had all the DLC packed in it.
I could be wrong, i often am. However my oppinion is not unprecedented. Home console Mario Kart titles have traditionally released approximately 6 years appart and Nintendo only usually releases one Mario Kart per system due to the game consistently being an evergreen seller (MK8 (WiiU) 2014, MKWii 2008, Double Dash (GCN) 2003, MK64 (N64) 1996, Super Mario Kart (SNES) 1992)
With this in mind its easy to imagine Nintendo not seeing the need to spend extra money making a new Mario Kart game that would only serve to canabalize sales of a game they no longer need to spend money on making.
Aditionally many successful games have recieved more then one "Definitive Edition" version. Street Fighter 2 is perhaps the gold standard for this practice with SF2 having something like 6 different versions (i looked this up on Wikipedia and only left confused)
Releasing further DLC for Mario Kart 8 allows Nintendo to further monetize its already large userbase, with the added advantage of profit margains being increased in the digital environment. Aditionally this allows Nintendo to eventually surplant MK8 Deluxe with a new content complete version without risking canabalizing sales or making the MK8 redundant by releasing MK9.
Finlly releasing MK9 in 2023/4 would potentially allow for gameplay to take advantage of increased hardware capabilities and it be a good obvious signal to consumers that it was time to upgrade to a "Switch 2" or whatever the current Switch revision is called at the time.