So this is how they're gonna do modular fluff, eh?
Maybe. I think it'll work.
What I'm afraid of most in this case is that wrought iron wall of tigers between the fluff and the mechanics. It looks like they're paying pretty close attention to it, to make sure that the mechanics support whatever fluff they're telling, and being pretty careful about it overall, so that's a positive sign.
But I can't help shake the feeling that different fluff should result in different mechanics. The specific mechanics should support the specific story. A jackalwere created by Grazz'zt should be something that, say, paladins can smite. A jackalwere just born in nature, maybe not?
But then, it sounds like their close attention is working to mitigate/remove this concern. Jackalweres have a gaze that puts critters to sleep...so it's not totally a bad fit for Grazz'zt, who is a manipulative little gremlin. So it sounds like their new stories are built from the mechanics: what other story could these rules tell?
Which is kind of cool, come to think. Sort of coming out the other side of it. A lot of old monsters have a bit of a haphazard selection of abilities because designers sometimes gave more thought to what the creature needed to do than to why it needed to do that (and jackalweres putting things to sleep is not a bad example of that). We can presume that the old stories fit these old mechanics "well enough." Now these old mechanics are also being translated into new stories.
That liberates DMs and individual settings to do their thing, too. Maybe Jackalweres in your game are evil servants of some god of dreams or something (sleep gaze!).
The mechanics-first perspective feels a bit awkward and backwards...but it is probably not a bad idea for an inclusive 5e, all told. As long as the new stories still relate to those mechanics, you'll still be supporting the story with mechanics.
...I imagine things like "alignment" fall more on the story side of things than the mechanics side of things for the 5e team, and I'd be inclined to agree, which can give us things like Lawful Githzerai -- they're the same critter, statwise, just in this particular story of the Githzerai, their monastic tradition has emphasized order and control rather than independence and self-determination (a la the 2e version).