The seatstay is bolted to the dropout, so it could be placed under another angle.
Nevertheless, maybe Miyata reasoned more or less the same at some point. For the 1995 year Koga Miyata intended to offer a carbon/ti frame with the ST setup. The 1st catalogues were already printed with the pre production model in it. In the end, the bike came never available to the public with the ST setup. I dunno the reason, but maybe it lay in the durabilty field. The Miyata bonds were strong though. From experience I can tell they hold up really well. If something went wrong it most often was unbonding in the dropout area.