I think the 18-50mm is an attractive proposition on any mound; yes, O.I.S. in any shape or form would be nice, but this is a fast standard zoom - you'll not need stabilisation most of the time. I recently bought the lens to use it on my Z 50 via the Megadap ETZ21, and I'm quite impressed - though ... more on that in a bit.
Anyway, I owned the 18-55mm f/2.8-4 when I had the X-E3; in fact, it was the lens I bought the camera with - and essentially for: as a zoomable companion to my Leicas. And in many ways, the lens delivered - but especially at the long end, I always found it somewhat underwhelming. It's not actually bad, though - it just renders a bit flat and dull at 55mm (you can correct most of this in post, but the lack of plasticity remains). The Sigma 18-50mm gives me very appealing results especially at its long end and otherwise certainly matches (or exceeds) the 18-55mm in terms of optical quality. That's no small feat - after all, the 18-55mm is well-regarded for a reason, and it remains one of my favourite standard zooms to this day.
Do I think the Fuji system specifically *needed* the Sigma 18-50mm? Not sure - but I'm pretty certain that I'd buy it again (or rather, swap it or have it converted) if it was ever released in Z mount. Among the convenience, it'd mean getting in-camera corrections (the ETZ21 doesn't transmit lens profiles, so you have to correct for distortion manually in post).
All that said, the fly in the ointment in my specific case is the quality of my go-to lens for the Z 50, the Nikon Z 18-140mm DX - while it's not a bright lens by any measure, its overall IQ is simply astonishing, and even though the Sigma 18-50mm is a very nice performer, it essentially doesn't top the much more versatile Nikon "superzoom" that's also not a whole lot bigger. I like using the Sigma 18-50mm and certainly don't regret buying it, it makes as nice a walkaround companion as the Fuji 18-55mm was and allows me to play with DoF in a very satisfying way - all in all, I think it's a really good EDC lens. But for dedicated photowalks, I'll keep picking the 18-140mm ...
Anyhow, I think there's a use case for a lens like the Sigma in every system, O.I.S./I.B.I.S. or not. Optically, it's a cut above most kit lenses, it has the constant bright aperture, is very portable and well made (though not as well as the Fuji 18-55 - that lens is a masterpiece in terms of build quality). If you're an APS-C shooter, this is a very attractive proposition, no matter what - and the price is right, too.
M.