I see two major issues with Samsung switching to Tizen: * Samsung is unlikely to get developers as talented as those at Google. I doubt that Tizen development could approach mainline Android's development pace, given the talent difference. * Tizen would need its own app store. Ask BlackBerry or Microsoft how well new app stores do.
Tizen is unlikely to catch up with Android functionality- or app-wise. Unless the carriers push it hard over Android, it will have no advantages and thus will not sell.