Its a very small percentage but when Yamaha is the market leader in saltwater outboards, the numbers add up. Thankfully, the OEM retro kits have come down in price so it's not as bad as one thinks. I also believe that the maintenance schedule for certain yamaha outboards have been "updated" to include an exhaust "replacement" at a defined interval.
I have owned 14 yamaha's from 150's to 250's and not seen the issue over the 7,400 hours of operation in any of my motors..... maybe I should buy a lotto ticket. who knows....
Since we are talking outboards, in particular, Yamaha outboards: very important for operators to understand the importance of using high quality fuel and Yamaha Ring-Free to keep the valvetrain free of carbon build up as that is the usual cause for premature engine failure.