Mainframe (zOS) software is still very much in use right now. Many of the most critical applications of banks, insurance companies, airlines, global companies, etc are running on (good old) mainframes.
However, for companies that are in the business of developing and supporting mainframe softwares, it may be (terribly) expensive to have the required infrastructure for developing, testing and supporting such software.
What are possible solutions for developing and supporting mainframe software without paying fortunes for the required infrastructure (and spending all software revenue to it)?
IBM is incredibly dense about letting small ISVs into the z/OS space. They fail to recognize that the entire System/360 and up ecosystem was built on the work of individuals. In many cases, they were systems programmers creating products to fit areas where IBM had not written software. They claim you can rent time at the Dallas Systems Center for $100/month but for me that's a lot of money. Their solution (zPDT) will cost you about $2000/year for licensing and over $1000 for a decent Intel platform to host it. There is also RDz (or whatever it's called now) but that's over $10K + $800/year.