Your logic seems to conclude that you keep paying for Netflix, but that your mom will be the only one to use it.
The Premium plan is 4k streaming, and up to 4 devices. But now they want to keep those two features bundled, while limiting your ability to stream to 4 devices (because now they'll all need to be in the same house). Sharing my account was my reason to suck that up and pay 20 bucks a month for 4k streaming.
Now only I can watch it, or in OP comment's case only his mom can watch it, so there's no reason to not just switch to the basic plan with 720p for 10 bucks a month. Point is, there will be cancellations, but also a ton of people halving their subscription cost. It's going to hurt Netflix on both ends.
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.