There are still downtown (including connected) hotel rooms both inside and outside the housing block available. I noticed that after the initial housing block rush, hotels actually started offering rooms on points again whereas prior there were none available, indicating the bookings were less than expected.
Google tells me there are approximately 7,700 hotel rooms in downtown Indianapolis, though I don't know how far "downtown" extends in that context. Going with an average of 3 people to a room -have no idea what the actual average is, just taking an average from "2-4" people per room- if every room downtown was booked like previous years, that would be around 23,000 people most likely there to attend Gencon or accompany someone attending for the majority of the long weekend (we will give the other 100 spots to unsuspecting business and family travelers not knowing what weekend it is).
That being said, that doesn't account for people who purposely booked hotels outside of downtown, attendees not staying in hotels (locals, vacation rentals, staying with family/friends, etc.), and day-trippers.
TL:DR- Going by hotel availability, badge sales so far appear to be slower than I would have expected.