If you view the mainframe as a standalone entity then its future is bleak. Using the mainframe as an enterprise server adds new direction and capabilities not available in the server farms currently in use. Undeniable mainframe (Enterprise) reliability, security and lower total cost far exceeds that of client server only solutions. Utilizing the capabilities of all resources can produce the best of service for all.
For handling millions and billions of data, Bigger clients are going for mainframe only. Now-a-days its not a single environment ruling. For webpage part .Net is good and for database side DB2 is the better one and J2EE is good at retrieving data from server much faster. So don't worry about the future.
We've got now very many LINUX implementation on mainframe.
According to that we've got some databases to.
Under VM or by partitions we can create very many images of LINUX
(on demand every client has its own system and database).
There is a possible way in a future:
very big data centers under LINUX on mainframes connecting by internet
or other network for small and very big clients whith workstation
or terminal only and peripherals(printers and like to that).