I think the launch of Chrome made the biggest impact on E, because Google's marketing made people who did not know what a browser was realise they had a choice.
For a marketplace to function you need information. When economists talk about that they most often mean price information, but the fact that there are alternative products is an even more fundamental requirement.
Also can people please stop comparing MS to Apple or Google. They do not have a monopoly to be restrained in desktop OSes: Google has been a target of the EU in search, and Apple has some pretty anti-consumer practices, they may not fall within the reach of current competition law.