I think that "don't borrow money on a declining asset" implicitly assumes that the borrower requires the loan in order to buy the car. IOW, they can't really afford the car, so they need to borrow the money for it. For people in that circumstance, the comparison of "invest at X%, borrow at Y%" is meaningless----because they have no money that they can invest at X%, Y%, or Z%. You can't compare two alternatives when one of them is truthfully non-existant.
For people who have adequate assets, it is meaningful. In fact, for a cash purchase they could consider it as "borrowing" the money from themself at an interest rate equal to what that money is currently earning. So the question then becomes "should I borrow from myself at 8% or borrow from the credit union at 3.99%?"
"A man who has a million dollars is as well off as if he were rich."
For people who have adequate assets, it is meaningful. In fact, for a cash purchase they could consider it as "borrowing" the money from themself at an interest rate equal to what that money is currently earning. So the question then becomes "should I borrow from myself at 8% or borrow from the credit union at 3.99%?"
"A man who has a million dollars is as well off as if he were rich."