Saurabh,
As you know,there are no guarantees in life...but most probably SAP will stay for another 10 years and MM will be there too, as it is a core module.
Even if companies use SAP MM (i.e. there is market demand), your employment and growth (monetary, non-financial) will depend upon the value you can offer your customer and your luck. Understand the main deliverables of a functional SAP consultant and improve your skill set. One thing that I have concluded is that it is not a get-quick-scheme but a long road to decent money and fulfillment (job satisfaction).
TW
added: business process knowledge and understanding is key for a functional consultant, these documents will help
success in SAP SD - essentials: business process
success in interviews - part 1: Process
Message was edited by: TW Typewriter - added in blue