There have been one and two person bands around forever. My organ teacher did a duo with a drummer and I saw many solo organists in the 60's and 70's. And there are always the pianists of any sort. The fact that arranger technology helps OMBs sound like trios and more is simply progression of technology. Even when using SMFs or arrangements, it is my keyboard playing that I focus on. I know that my playing could (should) stand on ots own, so there are no apologies for using arrangers in place of musicians.