If you look at the acquisition of Hotmail by Microsoft then you see a large amount of software implemented on open source software re-implemented on closed source software. Along the way, there were a few service outages and a notable security flaw where anyone could read anyone else's messages. That's a whole lot of effort to reduce portability and damage your brand. Furthermore, while you're re-writing existing functionality your competitors are adding new functionality. Note that when you re-write functionality, it doesn't guarantee improvement. Indeed, it doesn't even guarantee that you'll maintain the existing level of functionality.
In this case, you won't even maintain the current level of users, due to future concerns regarding accessibility and terms. So, would you either oversee the re-write of widely used code for dimishing users or would seek other opportunities, such as a start-up?