But twist is here they won't touch mainframes subject.
What does this mean?
Use this opportunity to learn all that you can. While you are doing this job, you are gaining experience (which is something most employers want/require).
So i scared about my future. Is it useful for my future or not?
useful for your future? Your question is not clear. Mainframe technology will be around for years to come - the name may be changed for political reasons, but the application inventories will continue to be supported.