Critical success is not the same thing as commercial success
I grant you that, it's completely beyond dispute, but the two aren't mutually exclusive. I think it's also safe to say that critical acclaim and commercial failure can in time give way to commercial success. I think the perfect example of this is the guy who has attached his name to this show, Judd Apatow. Freaks & Geeks and Undeclared were both commercial failures yet outstanding shows -- he persevered and eventually his name turned into the proverbial Midas Touch. It's not absurd to think she can do the same thing even if Girls flops, though it's already looking like it will be around longer than both of the JA shows I mentioned. Especially as long as she's capitalizing upon her status as the flavor of the week/media darling.
...and let's be real, you really think Lena Dunham isn't making bank right now? In October she signed a book deal for 3.5 million, plus whatever she gets off of Girls. Sure, she's not exactly an OT baller yet, but give it time