The simplest way I can show the error? If making a new piece of software is so expensive that it's far, far too expensive to do, how come folks enter the market everyday with new software which keeps replacing all of that stuff you think is irreplaceable? And they not only replace your stuff with better stuff, they do it at a fraction of the cost you take just keeping the lights on in your shop.
Math is not going to save you now. Looking at how fast features can be deployed is sticking your head in your ass. It's counting things for the purpose of counting them. It doesn't work like that. You may enjoy counting, sgraphing, and tracking points-per-sprint or feature-speed-per-team, but nobody buys or uses software based on feature count or team speed. It's not that these things aren't important. It's that you're confusing managing things with value creation.
I wrote an essay earlier this week about code budgets which I think can help a little by at least helping force conversations around the real issues involved. http://tiny-giant-books.com/1.html?EntryId=rec39SaDeZCZjauRo
But the larger issue here is that organizations don't know how to create and harness value. They're really good at hiring, managing, and a few other things. But those other things don't come directly into play here. It'd be great if they did, but they don't.
You don't know how to create value. Step 1 is admitting that. Without that admission, no amount of charting or graphing is going to help. And yes, you can't rebuild your software. Probably lucky you've got it up enough to provide value right now. I'd hang on to it.