For light web browsing, light word processing and email use, cloud for music, shouldn't 128 gb suffice over the next few years?
It would be enough. It's not exactly the same but I have an 11" MBA from 2010 with a 128GB SSD. It has Office 2011 installed. It even has Fusion installed with one Windows 7 VM just to do testing in IE. I have plenty of space left on the SSD.
As long as you are not collecting or editing RAW photos, video, or have a huge iTunes library 128GB should work fine for you.
128GB is fine for what you stated, though if you started increasing the size of your iTunes library it could start feeling a bit tight but keep that in mind given the meager size.