Weebly isn't out to replace web developers.

Weebly is more likely replacing low-end web designers (ie: the web designers who do projects for less than $5,000). But they are also growing market share- Weebly is cheap enough to get people online who otherwise wouldn't be able to afford a website.

If you are interested, I wrote a blog post examining the ways software is (and isn't) eating web design. http://www.sitebuilderreport.com/blog/how-software-is-eating...