it's not just a dev build, it's also a demo. who's to say that all of the work that gets done each week gets added to the demo instead of just a small portion of it? according to the decrypted files, there's supposed to be a couple of entire other characters. like, meaningful characters, and not just ps4-knight-bro. and they're old news, too. there may be a lot more of the game finished than we see from the demo, and it would make sense to not include those files in the demo, where they'll never get used (and will just be decrypted by tech-savvy users). ultimately, the point of the demo is to get you to buy the game (when it releases). there's no point in putting the entire game into the demo.
as for the "focusing on the game part", while as a porn game, porn first and foremost, it is still a game at the end of the day, and if the game sucks... then people aren't gonna wanna play it. certainly not buy it. even if the porn part is amazing. they'll just look up the CGs instead of buying the game, because the game part gets in the way. how often are people commenting in threads asking for complete saves so they can just view the post-game gallery instead of playing it themselves? so, yea, it's kinda important they make sure the game part is fun. I'm not a big fan of RPGM myself, but the updates to the dungeon has given me hope that the final product won't just be lackluster gameplay tacked onto still images/gifs I would've been better off looking up on e-hentai and saving myself the time and effort.
missing sex scenes is temporary, sucky gameplay is forever.