Old software is going to stay old software for a long time.  A few lucky organisations will be able to move their applications to new software, but most places will continue to use old software until they close down and no-one want to buy the asset.  It does not matter if the old software is secure or not, while it runs it will keep running.
If we think the problems of old software are bad now, we are just starting to see what will happen as IoT grows and you have some hardware device running software with little thought on how to make it secure and no ability to upgrade once it has been made.  Your grandchildren will probably still see embedded XP and NetWare while they struggle to cope with certification for SQL Server 2050.  
We simply have to accept that old stuff will always be around, and focus on what regulatory and business infrastructure is needed to support it.