There's dozens of original ideas Pokemon could use from nature and mythology.
They're not running out of ideas, they're just actively slacking off.
Or in the case of the sandcastle, trying to tie into the region unlike usual. This is the same region we get a lei, a wharf roach, an imported mongoose and a bird with a hula variant. They're obviously trying to give us as much "Hawaii" as they can squeeze in, and a sandcastle fits into that even if it's "eeeew inanimate object."
No, they are running out of ideas.
With Gen 1, Pokémon was a humble project, an earnest attempt from a young game designer to make a game he always dreamed of, with a new gameplay concept that he hoped would encourage social interaction (capturing, trading, and battling creatures across two versions of the same game).
Gen 2 was him trying to prove this is something that could stick around, by doing the same thing again, but bigger and better. Trying to live up to the lofty expectations set by the massive success of the first game.
After that, Pokémon became increasingly about the merchandising because of it's guaranteed pedigree. The games exist to sell toys and trinkets nowadays, and not much else.
When they make a Pokémon nowadays, it isn't usually because they want to do something creative with it. It's because they want to make something they can make an easy piece of merchandise out of.
Keychains, Teddy Bears, Sandcastles, etc. These are all things that are no brainers for some kind of merchandising.
Someone is going to try and argue with me about how Pokémon is pure and everything it does is done with the utmost creative integrity and thought.
But let me remind you that the new Pokémon game is being released with this.