Without having any second thought, I can say that SAP MM is definitely having a very good future scope.
Looking at the current growth rate of SAP technology, one would confidently say that 20 to 30 years down the line also It's going to grow in the same pace and if you are looking for a career in this field then surely you don't have to bother about the layoff in any way.
If you want to learn SAP MM from scratch, then here's the right video for you:
If you are more into reading, then do check out this material provided by Intellipaat:
Now let's see the various demand of SAP in today's world:
SAP MM, SD are some very popular functional modules which are in great demand. In fact, MM module is in use in most of the SAP projects as the material is one of the basic needs of most of the businesses using SAP. And, as per your interest, you can also go ahead with learning some more functional modules like QM, SD or technical modules like ABAP, Basis and so on to add to your current demand.
One of the highest-paid profiles in the SAP industry is Technofunctional consultants. So, the knowledge and skill of SAP MM can definatelly give you rapid career growth in the SAP industry.