You still need SLAs, you still need performance WAN circuits, you just may use less of your pipe, may have better performance across your WAN, and you may be able to downsize your circuits. But if it were my enterprise today, I would look at reusing some of those cost savings to even further optimize my WAN infrastructure and put in higher availability circuits.
WAFS is not a total panacea for all of your WAN infrastructure woes. Realize that one technology under the WAFS umbrella is typically not sufficient to meet every need in your enterprise. So don't approach your enterprise needs with just a WAN optimization approach if you're trying to replace or reduce the cost of your WAN circuits. So you need to look at all of your unique needs, including traditional enterprise applications, optimizations around those applications, the very chatty protocols, as well as just more general WAN and data transmission that can be optimized with LAN optimization.