there are a wealth of lenses made for the S35 18 x 24 sizes and then there's those made for the FF35 24 x 36. since these are readily available why not limit sensor sizes to suite them? sure, some of the FF35 prime and zooms may require a redesign of the mechanical housing to make them sturdy but at least the optics are usable
and proven. to redesign the housing would raise costs but it will be a lot less than having to built them from the ground up. already Carl Zeiss have released their FF35 Compact Zoom 70-200. it seems they are the only one who have envisaged this and are building their lenses future proof.