You will not normally get into a situation where something has to be downgraded to support a package, because most libraries are backwards compatible. This means that the real differentiation for OpenSource isn't marketshare or even marketability, but simply whether or not it can garner a following. Market share is the goal. The only difference is that the reward for more marketshare is different. There are also many kinds of software which have lifespans just too short to make it sensible to bind the customer in a service agreement.
Games are a good example.
Most games are onetime sells. In fact if memory serves, it's one of Eric Raymond's examples of software which should not be open source, at least not at first. But still not the ultimate computing platform. Desktop GUIs as we know them will yield to client-server UIs. People will be using a variety of gadgets, not just desktop PCs.