MS and Sony both have something they can really leverage against this streaming only solution. Local hardware in addition to streaming.
If they let you stream the digital games you purchase, then we can play at home at the best possible fidelity on the local console. And streaming will be a complimentary way to access the games we already own on the go (job, hotel, whatever). In a case like that I can't see anyone logically buying a game on Stadia when they can buy a game from Sony/MS where they can play it locally AND be able to stream it.
If they let you stream the digital games you purchase, then we can play at home at the best possible fidelity on the local console. And streaming will be a complimentary way to access the games we already own on the go (job, hotel, whatever). In a case like that I can't see anyone logically buying a game on Stadia when they can buy a game from Sony/MS where they can play it locally AND be able to stream it.