Sheikah Slate being upgraded from a Wii U to a Switch tablet is cute and reminds me of Pokedexes from previous gens being designed like Game Boy / DS. Now the Pokedexes are mobile phones which is kind of boring in comparison.
Honestly I think the mobile phone thing in Pokémon is one of the worst pieces of world building and art design I've seen come from the main series. It's always been that in the Pokémon world, the Pokédex came first due to its near universal usefulness, and the mobile phone got built on top of that. In our world, mobile phones came first, then the encyclopedias accessible on them came after.
That said, it does sorta make sense. Everyone realistically needs a phone nowadays, but not everyone, even in the world of Pokémon, needs a Dex, so it makes more sense for the manufacturing industry to pivot towards general purpose devices that happen to have Dexes, like how PDAs were displaced by mobile phones in our world, Dexes were displaced by Rotom Phones.
However, I would like to point out that the RotomDex is a tablet an awful lot like the Nintendo Switch! Pokémon also isn't the only series with these. The Koppad in Pikmin, the GameBoy Horror and its successors in Luigi's Mansion, the Tingle Tuner, Dee Ess (Island), Sheikah Slate and Zonai Slab in Zelda, and I'm sure I'm forgetting one.
I'd love to see the next Pokémon game take a step back and rethink the Dex. Like, actually let us ring trainers on command, the phone in Animal Crossing can't do this for some godforsaken reason when they removed the megaphone item. But design wise, I think, maybe a new, flat-sided Rotom Phone, and when you meet the professor, they add "Dex Modules" for cataloguing Pokémon. Including an IR camera, new sensors and new controls, that attach onto the side of your Rotom Phone. So it can "Switch" from phone to Dex.
Much more interesting than an app!