During 2012 and to 1H13, Gartner documented well over 100 distinct inquiries from clients who have been tasked with investigating the merits of moving away from their IBM mainframe environments.
Here is Gartner's analasys:
Follow a Multistep Process for Deciding Whether a Migration Off the Mainframe Is Warranted
Migration off the mainframe may or may not be the right decision for your organization. Infrastructure and operations leaders must understand the implications of key factors such as time, cost, workload, business impact, capacity planning, staffing skills and tools.
Read the full report at Gartner.com