I don't personally think Cortana will be on the other mobile OS's anytime soon for various reasons, mostly technical, legal, and due to competition. I do believe MS needs to watch what they say, and they need to focus everything on their OS's and accompanying services. If they make the mistake of going all in on services without the historical push for the OS, then they are doomed. Why? Because services can be duplicated and implemented much easier than an OS. If Android ends up being the OS that is used for everything, like Windows has been, then MS will loose the platform tie-in that has historically allowed them to dominate in the OS, Office, and IT services arenas. All it would take in the future is for one company to come up with a much more compelling set of services than MS and they could easily win market share if the OS is not part of the equation.
MS needs to push WP far, far more than they are currently. It needs to be their primary focus, and it needs to have the services FIRST, and it needs to offer more features and better integration of those services than what is offered on competing platforms. If they don't do this, then they will never gain more than a marginal mobile OS share, and this will also greatly impact usage of Windows on PC's and Tablets (more so the latter than the former). Cortana alone isn't going to win WP converts. Cortana with a great Office experience, superior enterprise/business support, a better and highly integrated Skype, and other features that are exclusive to WP, however, would. WP needs to have a number of key advantages that, when combined, offer a significant advantage over iOS and Android for users in countries where MS is still having a hard time breaking through, especially the US. It's about the platform in total, not just a couple of wow bang features.