There are a lot of places where siding out cantrips should be the go to. As an example, versus GBx, street wraith and Bauble grow their Goyf and are scooze targets. They tend to be a liability should you not see an Angler. As a result in this MU I usually cut all Baubles and bolts, I tend to keep Street Wraith in but I have been blown out by it more than once. Versus mill all cantrips can go. In matchups where using your Mana efficiently and holding up answers cutting serum visions/looting may be the go to. Etc.

There is no hard and fast rule that says cantrips should stay in. Yes, it artificially lowers your land count but Shadow is completely serviceable with seeing only two lands. You're right in saying that you should always cut the worst card before cantrips if possible. There are a lot of cases where cantrips are your worst card and end up being a liability.