This is an intersting observation.
I was thinking that SaaS always works in a platform based model. There is no client software that needs to be installed/or on-premise software to be purchased.
It is not always be the case. While SFDC provides a platform based SaaS model, there are other vendors who provide other models of SaaS.
For example, the vendor TwinField provides a Microsoft excel based add-on. This add-on needs to be installed on your desktop to perform extensive analysis on the financial data that is maintained by the TwinField services. The add-on interacts with the TwinField Services platform to retrieve the relevant data.
This is basically a hybrid model where you have little bit of on-premise and a lot of platform based services.
And this is absolutely the same architecture model that 'Composite Applications' follow. Its just that the TwinField does not implement this model in a Business Process/Flow context. Otherwise, it is very identical.
What I see is that, its not just the 'Services' that have the potential to be hosted in SaaS platform. Its also the 'Composite Applications' or some elements of 'Composite Applications' can also be hosted in the SaaS platform.