Marian, you're painting us as making an unreasonable demand - to house 100% of 200+ thousands attendees. I know you probably get frivilous complaints like that all the time, and it is likely very jading and exhausting. I do not envy someone in your position, just, please, hear me out.
What I've been attempting to convey is that housing is the single most important thing needed to determine if one can attend a convention. Even if you don't pre-register you can walk in on Wednesday and get a badge. Housing alone will cost the attendee between 4 and 12 times the cost of a badge if they don't live locally or have someone to stay with.
Ignoring cost for a moment, as brotherbock had previously stated, for some it is imperitive that they be next to the convention center, be it mobility issues, needing room access to do things like use kidney dialysis bags like my father inlaw, etc.
Knowing there are possibly 200+ thousand people attending, it is simply prudent and responsible to try and ensure one has a place to stay for a 4 day convention before paying to attend said convention.
The inability to view current housing options before buying a badge is a hinderance to those who desire any level of security in their arrangements. Being locked into an "administrative fee" just to see if the housing you require is available still is unkind.
How would you feel if you budgeted $1000 for a vacation, and they charged you $100 just to see if there were any flights available, and then cheapest you could find was $1200? How would you feel if you required a handicap accessible room and had to pay $100 to see if any were available and there weren't any?
It felt pretty bad at my end. And, while I was able to work things out in the end and have experience I can draw on for future endeavors, I wanted to provide you with a suggestion that would prevent others from feeling negatively towards their experience, and prevent potential harm to those who would lose out entirely.