Let us look at what Linux considerations can change the TCO equation to your benefit.
6.3.1 Opportunity factors
The importance of nearly 100% availability may argue for many projects being done in z/OS. However, when fast time-to-market is essential, Linux on the mainframe may come into play because for some applications Linux can have adequate availability characteristics while giving you the following:
Time-to-market has become one of the most important attributes a company can have. If the IT infrastructure is impeding development (or whatever is important to your business, such as marketing), it can be costly in lost business. If this is the case, you might want to consider Linux. The benefits of Linux can be summarized as follows:
Another kind of time-to-market is the possibility to provide new images quickly to the customer by using z/VM and cloning techniques.
Relevant part of book: Part 5, "Running Applications."