I'm not sure I agree. If the choices are:

I can keep my account and downgrade my plan and then pay for a separate account for my Mom, then they would make about the same or even less money. They certainly won't be getting any ad revenue from either account.

I can cancel my account outright and get my content through alternative methods where Netflix is not compensated.

It seems to me the choice is obvious here and the calculus doesn't swing in Netflix's favor. Perhaps for the majority of the subscriber population it will, but that's a bit of a gamble when my generation and really every other generation after grew up knowing how to pirate whatever they wanted.