It actually did pass up 40,000 plugins already, but 11,500 of them have been removed for WP.org violations like security, copyright, GPL, and other reasons.

I guess you need some count to celebrate though, and the active count is certainly easier to track. For the record though, the 30,000th plugin approved by the WP.org team was actually Google Content Experiments, added back on July 20th, 2012. The 40,000th plugin was WP Justclick, added on December 30th, 2013.