I think it's more that the 60k number was dangled before them early on and now their hearts are set on it for irrational reasons. I probably fell into that category until recently, too, but having read the opinions of other posters, I think better points have been made by those arguing that something in the 50s would be just as good, if not better. In addition, posts like "anything less than 55k would be a disgrace" convinced me to rethink who I was in agreement with. I haven't read a single post in this thread that made a convincing argument as to why building a 60k stadium would make us more competitive in any way that actually matters.
I wouldn't be stunned if we built a low-50ks seater stadium on BMD, allowing us to better preserve our atmosphere and the fans of clubs in larger stadiums ended up envious of us rather than the other way around.
Either way, I think these fans just love complaining and would, without even the vaguest hint of irony, be the most vocal in their anger if we did move into a 60-65k seater and the atmosphere ended up like the Etihad/Emirates.