I've been on a couple of successful migration projects, both of which moved things to unices. One was motivated by the $5 million/yr costs to maintain the mainframe, which was replaced by a handful of lowest cost bidder unix boxes. The second was regulatory-agency-mandated cleanup after a financial fallout, and unlike the previous two or three times that company had tried and failed to consolidate their risk reporting onto a single system, this time it worked. Because it had to. The threat of an external regulator was sufficient to break down the previous barriers of balkanized system owners protecting their turf.