If one can devise a set of tests exhibiting that a particular instance works correctly, then a subscription becomes possible, as in "the customer pays a rent, a

If one can devise a set of tests exhibiting that a particular instance works correctly, then a subscription becomes possible, as in "the customer pays a rent, as long as the software works as specified", maybe even along with some provision for penalties. In a way that's what xaaS offers. However devising a way to let the customer keep his data on-premises may be a major factor, as some (many?) consider their data confidential and therefore prefer store it on their own controlled environments. As any serving software needs an adequate underlying machine and OS, the service company needs subcontracting partners and/or ways to certify. It seems to me that Red Hat (along with SUSE, and many other similar companies) may one day adopt this approach.