It's only when demand is completely out of hand, or when you don't want to Osborne yourself, that you want to stop advertising.
It's designed to get the guy who has a Tesla and a Tacoma in his driveway and who sees fullsize domestic trucks as a penis extension for backwards hicks to trade both those in.
This isn't about converting existing owners. It's about growing the potential market.
This article shows us how many new customers Ford is attracting: https://www.motor1.com/news/552814/ford-lighting-customer-ne...
There are a lot of F150s though, and the Lightning’s design is distinct enough that it advertises itself even with a tiny fraction of owners actually converting.
Also, the ridiculous things people are going to try towing with that much torque will win some over as well.
[0]obviously hyperbolic but fleet electric makes so much sense because many are already centrally parked and the rest can be trivially recharged at home much easier than dealing with gas.