From discussions with a few of the Cisco SEs that I know, it will be a little while yet before ACS is canned; indeed, ver 5.8 has just been released so that's got at least 3-4 years before it becomes unsupportable (ver 5.7 has just been marked as EoS - http://www.cisco.com/c/en/us/products/collateral/security/secure-access-control-system/eos-eol-notice-c51-736015.html )
As others have said, the intention is that ISE will become the de-facto AAA product for Cisco but they have a while to go yet before it's ready to do the job of ACS. We run ISE 1.4 for end user enforcement and ACS 5.6 for device access and migrating isn't even on our radar.
Regards,
John