Thing is, we're not going to be losing any dreadnoughts to piracy. Against a corvette, a pirate has a decent chance of winning. Against a dreadnought, not really.
In addition, you're incorrectly interpreting patrol route-main battle dynamic. With dreads, we have the ability of drawing in our piracy patrols in favor of greater combat strength. With corvettes, that option is simply not there.
Dreadnoughts offer better flexibility and better performance at a slightly greater upfront cost. In the long term, they're clearly superior, and we've already done most of the hard work in building up a dreadnought fleet.
Okay, so it's a figure you basically made up.
Pirates are attacking our trade with Autochtonia, so I could just as easily argue that we need just 1 ship to deal with them, to secure the trade routes that are actually vulnerable to pirates.