Civil, Mechanical, and Electrical engineering are considered as mother of engineering. In other term, these are core branches.
Now coming to Mechanical engineering and its future.
Look around, to any non-mechanical engineering area, you will find mechanical engineering usage.
Take Civil Industry, which is growing fast with help of heavy, more precise mechanical equipments, like earth movers, drillers, cranes, etc.
Look at electrical industry, which is also heavily dependent on mechanical industry starting from power generation plant machinery to distribution network.
Look at chemical industry, which is also dependent on mechanical industry in terms of chemical factory setup, equipments, etc.
Even you take medical industry, in which robotic operations, diagnostic machinery are mechanical equipments. List is endless.
Now coming to future of mechanical industry !!
The future lies is merging of multiple industries to create new and innovative solutions. Mechatronics is one such example in which mechanical engineering works with electronics. IOT is a combination of IT, Electronics and Mechanical industry.
Did you see a common thread on all evolving areas, may be robotics, may be space science, may be medical industry, may be IT, or anything else, all have a walk with mechanical support.
Therefore, if you are a mechanical engineer, be rest assured, it is going to penetrate in all industries. You can make a career of your choice while being in mechanical industry. There is no end of possibilities of new solutions.
At the end,mechanical industry is bound to grow to a limit which is not yet defined. The only limit exist is in your mind.
So keep your mind and interests open and keep exploring new solution which you may create for mechanical itself or other industries.
Good times ahead! Enjoy!