One of the overarching principles of Plan S is its commitment that funders do not support the hybrid model of publishing. One reason for this is the reality of double payments, where a single journal provides the publisher with a revenue stream both from full subscriptions as well as from APCs.
Another reason is that although hybrid journals were perceived as a bridge, by which publishers could move their model away from subscriptions and towards open access, the reality is that this has not happened. See cOAlition S news item ‘Why hybrid journals do not lead to full and immediate Open Access. In short, hybrid journals have demonstrably failed as a viable transitioning strategy towards full Open Access.
Transformative Journals differ considerably from hybrid journals in the following ways: