On the topic of reasonable alternative tools, Sketch[1] has started to gain a tremendous amount of traction within a short amount of time within the web design community.
As a Sketch user myself I still recognize downside for some is that it isn't cross platform. There are still a lot of people that have to use creative tools on Windows for one reason or another. Data exchange is also a big deal. Sketch does make an excellent alternative to Illustrator for my purposes.
I'd argue that alternatives to Photoshop and After Effects are the killer apps needed for getting most people out of Adobe's ecosystem. The OS X specific alternatives for Photoshop (Acorn, Pixelmator, etc) just aren't quite there yet. I really, really want them to be though because I've held off on upgrading from CS4 for about as long as I can.
[1]: http://bohemiancoding.com/sketch