How to argue for Unix vs. AS400/CICS

As a traveling consultant, my road took me to a massive Vegas Gaming operation, that runs it's shop with AS400/CICS. The legacy-4 rooms size- computer is protected more than fort Knox, and the level of trust that I earned is envious to many of the top leaders of the organization, and beyond.

Yet I see the Legacy system holding back many opportunities for success, compared to what UNIX can deliver. Fear of change + job security + paranoid security + Grey hair + Tough-guy-gambler image, keeps AS400(system i) /CICS running, with problems building up to a staggering level, that Unix can solve in a blink of an eye.

What line of thought you can say, that can be a good argument for Unix vs. AS400?

Take your time. Thanks for reading.