“Which market segment is the one that is totally saturated with digital signage five years from now? (Is there a market where the gold rush is over and every new opportunity has been already taken?)”
It appears to me that the current gold rush is happening in retail and that it will be the market segment most likely to be completely saturated in five years. We are already seeing the installation, and sometimes subsequent removal, of various pilots and test installations in retail including interactive touch displays, facial recognition enabled customized content, and self-service ordering. To me, this usually indicates that the segment is already nearing maturity and retailers and digital signage companies are searching for any continued growth. While I would not argue that the segment still has room to grow through innovation, I believe this will only occur in the top-tier retailers who have the volume and the margins to afford the technology and drive the ROI.