Return on Investment is a driving force behind Service Oriented Architecture.
By creating a platform for interoperability you create the opportunity for re-use.
Re-use is the key to your ROI. So SOA should embrace the heterogeneous nature of IT and must wrap and re-use existing IT assets not remove and replace.
This not to say that you can not use this to sunset older Applications, but the purpose is to create the ability to discover data, processes and services for new and innovative uses.