Nintendo treasure and producer/director Shigeru Miyamoto had a few things to say about adventure games in a recent company Q&A.
When pressed as to why Nintendo doesn't continue to support the genre in the modern era (with Famicom Detective Club used as an example of the company's legacy), Miyamoto responded that adventure games require a great deal of work due to the globalization of gaming: most notably, translation concerns.
Speaking to investors, Miyamoto notes, "As for adventure games, I’ve made a lot of them, starting back with Famicom Mukashibanashi: Shin Onigashima (Famicom Tales: New Demon Island; direct translation of the Japanese title), but the environment for production is more demanding these days. Games today are localized in 10 or more languages, so the cost of localizing the voicing and script for an adventure game (which generally has a great deal of text) is enormous."
He also plainly states the genre's waning popularity (noting that "older games like [him]" are the main fans), while citing Ace Attorney and Professor Layton as strong recent champions. With the closing of Telltale Games (which wasn't entirely related to the stagnation of the genre, but likely contributed to it) there's even less consistency in terms of releases now.
Miyamoto concludes by telling fans to "not to give up hope," but wants people to understand how challenging the whole prospect is in 2019. Anything on that front may come from Nintendo partners: Miyamoto doesn't confirm that anything is in the works from Nintendo, but name-checks Next Level Games (Luigi's Mansion 2, 3), explaining that said hope could come from a studio close to the publisher.