Why mobile web first

This is something I've said in summary form a few times lately. I use this (almost exactly) over and over for clients, but like a lot of stuff I share, it's not secret and is universally applicable:

A mobile web presence is simply required, for a few key interfaces, for marketing and for fallback in case of application failure. But instead of a requirement, the mobile web can be another opportunity, and another platform to access even more paying users of the service.

• Universal access – Everyone with a browser can access it, from anywhere. The rise of smartphones will be joined by purchases of increasingly data-aware featurephones, and even with more information workers, provided with connected desktop computers. Access for everyone is just another opportunity. 
• First step online – The web is well-understood, easy to explain, and not constrained. Those who start out on one website rapidly begin to use the whole web as an information source. It is less of a reach when marketing new services to link to the web, than explain an app. 
• Gateway to apps – Websites are low-risk for the end user. They can try it, and if value is clear, can be more easily enticed to download an app, if one is available for their device. This can even be extended to an upsell for apps if their device does not support it; the user can be additionally enticed to get a better device sooner than they might otherwise. 
• No publishing – Changes can be made (almost) immediately. Especially when launching a new product, and testing the waters with users, bugs can be corrected and user feedback added to the product without the long cycle of publishing and download from app stores. 
• Analytics – Even without any other reasons, one immense value of mobile websites is in tracking use in real time. Not only can trouble or opportunities with features or behaviors be spotted, but platform usage trends can be detected. Strategy can be rapidly changed to devote resources to the apps that need it the most. 

Okay, not universal. Nothing is. But all other things being equal, or as a starting point to evaluate the other data you have, I tend to assume you start with the mobile web, or at least include it heavily in the release of your app.

The mobile app you are launching on multiple platforms. Which you decided on by checking what the end users actually have, right?