We too use our Y-15 every day and only rinse in fresh water when we go up a river somewhere! This motor
is now 7 years old and still on its original impeller. I carry a spare around, and one of these days soon I'll change it... hopefully before it fails in an awkward place. OUr previous motor
was a Suzuki 15, used similarly, and it's original impeller lasted 8 years and then crashed.
So your rate of failure seems high to me. There may be a difference in the water pump design in the smaller motors that reduces impeller life... who knows... but I guess that preemptive changes may be the best way to keep you out of trouble.