Atheism grows because as religion increases it becomes more and more commercialized to the point of having no more credibility. The reports of heinous crimes against children by those who profess to be spiritual do not help either.
Yes, extremism can live anywhere and has at one time or another. There are and have been extremist Christians, Moslem, Buddhists, and now atheists. The reason is they are all human and subject to our frailties and prejudices. So, although I'm not afraid of the vast majority of any of those groups, there are some extremists in all of them to be feared.
Insufficient data. Atheism is simply a lack of belief in a religion. What ideals occur along with this spread are a critical factor. We could just as easily be talking about the world benefiting from the spread of a group of humanitarian minded people as the return of groups blindly following someone like Stalin or Mao. The most likely scenario is somewhere in between I think.
I guess what I am asking is indirectly will there be more benefit from less people getting involved with religion but your comment is true, who knows the future. We have reached this point with most of the world having a religious belief but some religions have gone too far which has given rise to atheism.
I think you have science to 'blame'. We have made great discoveries and it has led everyone to think differently about what they've been told about how we got here and the answers of the universe. People want science to backup claims not just faith.
I am willing to say this, a shift away from religion would leave a large segment of the population less vulnerable to dangerous ideologies.