“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?)”
I can see two markets that could potentially be saturated five years from now. One would be clinics and medical waiting rooms. This is because you have a captive audience. If you provide signage to occupy customers for the duration of their wait, you could have really good return on the amount of views received off of those signs. Another area that also has a captive customer base is gas filling stations. These small screens are starting to appear in gas stations around the area, but what better way to entice customers to come inside for any of the current sales occurring on food and beverage products. I foresee that, five years from now, both of these markets will have digital signage throughout all locations.