mainframe -> minicomputer -> PC -> smartphone

It's hardly a sideshow. If you find the freedoms provided by free software valuable, Software-as-a-Service is basically one huge step towards an even more proprietary model than we already have. People obtain their accounting software, their CRM, their word processors from these services, which at the moment deny some of the user freedoms free software explicitly tries to defend.

If we thought older iterations of MS office propagated vendor lock in, what about not being able access the software because your subscription to the Office Service has expired? The concept of "legacy" software becomes redundant, and users are forced to pay annual fees regardless if they feel an upgrade is valuable.

As for mainframes, these aren't the sort of services mainframe providers were renting out cheaply to the average consumer, are they?. You can't separate the technology from it's price to provision and it's accessibility to general users. "Cloud Computing", whatever you may think of it's technical foundations, is relatively new in terms of it's ubiquity and the way it is being targeted at the general public. If people just accept the closed, proprietary nature of these services without question, then we will see the erosion of basic free software principles.

If you want a decent example of this already happening, have a look at Dropbox. It's security is considered to be quite average, user private keys are accessible to Dropbox staff, and it has been shown that staff have used keys to look into user files. Despite this it is probably one of the most popular storage locker services, because it got in on the ground floor and users have just accepted this way of operating. People who move most likely end up at another service which offers essentially the same proprietary service, just run by someone else.