People hoping for a disaster probably won't get their way. The Stade Franšais - Ulster match organized in Brussels a couple of years ago sold over 30,000 tickets before it was cancelled. Combined with the tickets that would have been sold on match day, I'd call that a big success. I don't know why Saracens - Racing Metro would do any worse.
I'll certainly be there. Just like last time, when Brussels was covered with 10 inches of snow.