I think we may be merging the openness of source with the openness of formats here. I can certainly see your point about proprietary formats, but it becomes less compelling when proprietary software stores data in open formats, as the next version of Microsoft Office will. I certainly can't speak for Microsoft, so I don't know whether they intend to address concerns like yours with this move, but to my mind it does change the question a bit.
On open != free, you're right- I misread your article. I'm sorry to have implied you said open source software has no cost when you clearly referred to the cost.
One other point- in the case where a project stagnates and a single developer picks things up again, I may have trouble trusting the code. It's a sad statement of the times, but I'm much happier installing software from a group I may have heard of than from some dude. While I could go look at the source to verify it's not doing anything sleazy, that doesn't practically scale. In fact, I know people who want to do exactly this but can't because their employment contracts forbid it. Their employers are too afraid of opening themselves up to legal action. (to my mind that's another sad statement of the times, but what can you do?) At any rate, m hope is that eventually code signing will be prevalent enough that I can count on some other agency to figure out whom I should trust.
The prospect of Microsoft cutting support for Office is no different from the prospect of the developer community stopping work on any particular open source project. As a customer, are you going to get in there and start coding? Maybe you will if you're qualified to do so, but that's a cost to you; hardly free. More likely you (like most of the world) aren't a developer, so you're out of luck. You either get to convince/pay someone to help you out or switch software. Free software is immune to risk? Sure, just like Macs are immune to viruses.
Best Open Source Software for the Macintosh
Best Open Source Software for the Macintosh