Ryanair have adapted their boarding calls to include a bit about the reserved seating and the website has been up to date with the new process for a while now.
I would be annoyed if, having paid extra for the exit row seat (which many airlines charge for, both full service and low cost) to find someone who hadn't paid sitting there from the time of boarding. I would not mind so much if someone moved there on instruction from the crew as they would have just got lucky.
I do wonder how it works on a totally full flight though. If no one purchases the extra leg room seats then eventually someone will get them. I would be inclined to get myself to the end of the boarding que to eventually be "forced" into taking an exit row seat at no charge!
As for people paying extra to get priority boarding then that's up to them, but no one was ever guaranteeing them the exit seats when they paid for that little extra. All it states is they get on first.