I don't have a problem with demon-worshipping gnolls. What I don't like is the creation of gnolls by a demon being presented as fact rather than faith.
A bunch of gnolls who believe they were created by a demon lord and seek to emulate that demon lord is cool. Saying it's true just seems really lazy. I have just as big an objection to the relationship between elves and Corellon whatsisname, orcs and Gruumsh, and similar cases.
It makes sense that elves are commonly taught a given creation myth, and orcsn and gnolls as well. But I really don't like making those myths literal historical events.
5e is trying hard to appease people who don't like D&D alignment, but at the same time baking alignment-based concepts onto various creatures and setups. It's an unfortunate mix.
It's not a lazy story, it's just not a story to your liking. Which is fine, it's certainly a change to the "gnoll story" from earlier editions, even though the demon-worshipping part was added in 3E.
One of the reasons (IMO) they did this was that . . . we have plenty of evil humanoids in the game (orcs, goblins, etc) and gnolls were under-utilized. So, they got an upgrade in 3E starting with the Chainmail miniatures game, and that story became popular enough it "took over" gnolls. 5E certainly takes it even further than 3E did.
One of the things WotC, and the gaming community, is wrestling with is presenting classic "bad guy" races without the baggage of racist thinking/language. If gnolls are "people", sentient, capable of free-will . . . and they are all crazed demon-worshippers . . . that's a problem. But if gnolls are demonic creations of Yeenoghu incarnated on the mortal plain, they aren't "people" but "monsters/spirits". Is that better than the old story? YMMV.
One of the more interesting takes on gnolls is from the BECMI D&D era, and the Mystara campaign. These gnolls were related to other dog-headed humanoids like lupin and hutaaka (yes, hyenas aren't dogs), but were as savage as the typical depictions of orcs and goblins . . . . for the most part. There was a tribe of gnolls forced into caverns under the Sind Desert due to an environmental catastrophe, where they met a tribe of elves there for the same reason. The two cultures started cooperating, and eventually formed a shared, dual-culture, Graakhalia. It was pretty neat.