Ah yes, I forgot about Oricon not thinking biblia as light novel. Just found an article at BT forum from Teh_Ping, biblia's 3 volume sold more than 4 million, which mean more than 1 million average per volume, and MW bunko print 800k copies as a first print for vol 4. I wonder if they will make an LN version of this like Hyouka to sell even more copies
But back to the topic of Index, I think the biggest think that hold Index from getting more new reader(and also bigger sales per volume) is the number of volume in the series. I mean, as a new reader, it's hard to pick the series when it already has 29 volumes and a new one every 4 months