“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 don’t think there will ever be a market that will be totally saturated with digital signage. Retail will always be king. I bet there is a retailer out there who is planning to have a retail location where all the walls, ceiling and floors are all direct light LEDs. Gaming and hospitality will always be slightly behind retail. There are dollars to spend on digital signage, but the adoption always seems slower. Education and transportation will always be behind because funds for digital signage are usually at the bottom of the list.
I still think the area of least emphasis in digital signage is effective content. No matter how much money you throw at the hardware, the entire installation will be underwhelming if your content is lacking.