I know. When we release a newsletter, it goes to about 250,000 boaters. Scores of them replied to say the same thing. I understand the physics about centrifugal pumps. And in a purely theoretical way, it shouldn't shorten life. I agree with you.
Now contrast that to a yard full of mechanics and boat builders from 4 yards. My daily contacts were with guys having 25-45 years of experience helping me understand why 20% of the bilge pumps on Red Head had failed. They made it clear. A lifelong customer comes in with a burned out pump. They replace the pump and in a couple of years it comes back, burned out again. Eventually they instead remove the 3/4" reducer and put in 1-1/8" hose along with a 1-1/8" thru hull. The pump never comes back for replacement. Similar things happen if they reduce the size of the pump to something spec'd for 3/4" outflow.
They told me how it's a very consistent finding. So you can argue the physics of centrifugal pumps all day long. In the end, the pump burns out.
Yogi Bera explained it best:
"In theory, there is no difference between practice and theory. In practice, there is."