As the SO is growing the number of users with more than 3K rep is also growing. I believe that because of this even some valid questions are getting closed simply because there are too many people with the power to close them. So shouldn't the bar be set a bit higher (say 6K reps) to keep these close voters at bay?
No; with the same reasoning, there will be more people to vote for reopening questions.
Moreover, as the number of participants in SO grows, the number of posts grows proportionally. Ultimately, the ratio of users to posts will not change significantly.