Done this post before, but it doesn’t hurt to re-iterate.
High speed (breaking wash) causes a disproportionate amount of damage (bank erosion) than slower speeds (no breaking wash). 4 mph is a maximum not a must. On wider canals that's fine, on narrower canals 3 mph or less should be the order of the day.
Whether or not your wash breaks is not just down to the speed you normally do. As you travel along the width varies as does the depth. The narrower/shallower the canal the lower the speed at which your wash will break.
You may be travelling at a speed that doesn't break wash if you travel in the centre of the canal but if you meander to the side then your wash will break when it didn't whilst in the centre. This is because by moving closer to the bank you have effectively narrowed the canal on that side and of course the water is shallower closer to the edge.
Now there you are driving your boat keeping a good look out forward how can you be expected to see what is going on behind you? Well if you stopped steering your boat with your bum you might position yourself in such a way that you can easily look back from time to time to see what is happening with your wash and adjust accordingly (its part of being a good boater).
More especially at this time of year many birds are nesting often at the waters edge. 4 mph can swamp a nest of eggs or drown chicks. You , therefore, are a murderer. Yes I know the canals were here before the wildlife but then the birds were here before you.
Not all fish manage to get out of the way of your propeller. I often see fish with a prop gash floating on the water. Some fish are shredded so finely by morons using too much power to move off or stop that the remains never get to float.
Now when tied up my boat rarely moves (fore & aft) anyone wanting to see how this is done is welcome to stop on their way past and ask. But there are times when rings, bollards, or firm ground are not available. You can argue all you like about mooring on soft ground but at the end of the day some visitor moorings are posted where the towpath is soft (the 14 day moorings at Shipton are along a soft towpath). I don't give a monkeys what you think, or what common sense says, people moor where they feel its nice, where they feel they can enjoy the canal or the local pub or, provided you haven drowned them, the chicks.
The canals have changed their use, they are no longer a trade route, but a leisure park. They will never ever return to being a trade route however much the Greenies might want it. People come to the water for the peace and quiet. They come to slow down and chill. If you want to go racing around might I suggest you get a bus or hire a car. If you have to get somewhere in a hurry might I also suggest you get out of bed earlier (lets call it better planning). Whilst you are entitled to be on the water it would be much better if you paid attention to the environment and inhabitants of the canals and stopped abusing yourself with the tiller handle. Who knows you might even start to enjoy it!