Are you okay with Amazon taking your code and setting up their own version of SaaS for it? Not saying it'd make financial sense for them, just that… once you open source it they can do what they want.

Also remember that open sourcing something isn't a marketing panacea; you can still struggle to get people interested in it afterwards. It's a useful step if it calms your customers since they don't have to worry as much if you go away, and your value isn't the actual software but support around it or something along those lines.