You know, pointing how this pairing is popular on Tumblr kinda makes your argument invalid. (F-F-Fires shot!
)
More seriously, Bianca is one of the main reason that ship will never float for me. Varric is not ready (and most important, not willing*) to let her go. Whoever would fall for him would be stuck in Bibi's shadows, and that is not a fate I wish for Cass - or any woman, really. Cass deserves someone's unconditional love, nothing less - she lost so many of the people who were important to her (when you think of it, Galyan, her first (and depending of your Inquisitor, maybe only) love, was killed by the guy Hawke and Varric set unintentionally free), it would be cruel to force her to play the second fiddle to Bianca.
*ORLY? YRLY. "Do you think you'll see Bianca again?" "I always do". Varric may be pissed at Bianca for the whole red lyrium thing, he's no intention to stop loving her. I know plenty of people have argued their Inquisitor should have been able to talk Varric out of the relationship, but you don't force someone unwilling to hear to listen to reason. No mention the Inquisitor is not Hawke, aka Varric's best friend and his almost-brother/sister. Varric is even nervous to actually consider the Inquisitor a friend. Hawke may have had a small chance to talk to him, but not Quizzie. And definitely not Cass.
Anyway, this is why I'll never join the Cass/Varric topic. It might be all love and suggar and unicorns, but no one loves a party pooper. 