While there will always be market leaders we need to break the stranglehold that a few companies have placed on the technology industry. Just as chestnut blight only attacked chestnut trees there are viruses that only attack computers using a specific operating system, i.e. Windows, Macintosh, Unix. Imagine now a virus that can infect all computers and all systems. This is the same as a variant of chestnut blight that not only attacks these trees but, in theory, kills ALL trees. It is diversity that keeps a scenario such as this from happening.
All trees are different, even among the same species.
Some trees don't die from blight while others do. The same can be said about computers. Even simple differences in configuration can render a system immune to a particular virus. Open source software, with its diversity, preserves a small amount of protection. I consider it the same as farmers planting a variety of crops so that one failed crop will not wipe out the entire farm.