A BlackBerry lurker here. Well, BB10 runs Android apps, but not all of them, and without anything resembling integration. I have to use an old Starbucks app because the most recent one doesn't work without Google Play Services. If MS decided to install a runtime, it would have the same problem. The runtime hasn't helped BlackBerry increase market share, and it won't help Windows Phone.
Universal apps are the best option for MS to grow adoption rate. No, Starbucks won't develop a WP10 app unless WP10 gains at least 10% market share on mobile devices. A glowing apple is still the most popular accessory for a Starbucks cup. But a wider acceptance of universal apps, combined with developer support for the idea, will eventually make the app ecosystems platform-independent, and that will be a good thing for everyone.