There are a LOT of things that don't change over time.
If it works it works.... As for the impeller sure it's a good idea to change yearly but I had a '59 Evinrude 7.5 25 years ago and a friend has it now... To the best of my knowledge it has the original impeller in it as my school teacher who gave it to me said they had never done anything to it. The impeller in it is at least 30 years old now and my friend is still using it.
They aren't THAT bad of a design....
As for the flow of water... The water isn't picked up from the movement of the lower unit through the water... It COULD be if the intake were designed differently but only at high speed... Low speed operation which doesn't necessarily mean low rpm would always require the work of the pump.
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