Earlier quoted context omitted.
Their point is that people do pay for these services and they have still introduced ads to squeeze out more revenue. Ergo, paying for a service guarantees nothing.
What you are describing is called "ad-subsidized", you pay less than the full cost and get ads to cover the difference.
Becoming a paying customer is just a negotiation with advertisers to raise their rates until Netflix concedes to what they want. Since they know you have more discretionary income Netflix can also charge more for ads. They're also incentivized into turning those customers into ad supported by increasing the no ad plan cost.