Exactly. I have one of those damned things, 70s vintage. Every time I look at that plastic gizmo in the bottom of my boat, it gives me the willies! Going to plug it up permanently. With GPS, these are pretty much obsolete. If I ever need a mechanical knot reading, will use the old knot tube (or a knotted line:-)
The nice thing about a mechanical speed sensor is that one can see the effect of current on the boat.
GPS give a great SOG, yet.... there is no information about current.....