We have 11 Sigma lenses so far for FF mirrorless, all of them "stretched" DSLR lenses. As Bernard says, the advantages to these stretched lenses over an adapted lens are minimal. It does eliminate one mount, a place where the connection could be loose, not quite flat, or both. It could also eliminate an electronic translation - assuming Sigma put a chip that spoke native Sony or L-mount in the stretched lenses.
What a stretched lens does not do is allow the more innovative lens designs we've been seeing for mirrorless...
At least to me, a stretched lens is a very close relative of a lens on a very high-quality first party adapter (Nikon lens to Nikon body). If you count the 11 stretched lenses for L-mount and FE mount, I would say that you have to count all Nikon-manufactured F-mount lenses that are fully compatible with the FTZ as Z mount, and all Canon-manufactured EF lenses as EF-R. I would say cross-manufacturer adapters are a little different.
Maybe there are three categories of lenses - true native lenses, stretches and first-party adapted lenses (together), and lenses that were never meant to go on that body but have been made to work.
11 Sigmas so far, 7 (supposedly) to come for L-mount this year. Will they be 7 more uses of the lens-stretching machine, 7 new designs, or a few of each? If they are new designs, what mounts will they come in?