It's just like cable TV. They promised that a subscription to cable TV would mean no ads. They lied, and went back on their promise within a very short time.
They'll do the same here.
20 years ago, you'd be right. This argument would be paranoid nonsense. But that's not the world we live in.
We live in a world where your car manufacturer wants you to pay monthly for heated seats. Today it's normal for hardware you bought outright only works if you pay rent to someone else, and services like Netflix reduce features and increase the price. Amazon can, at any moment, delete books from your kindle that you already paid for. Digital rent extraction is a huge part of how the tech industry operates today.
Yes, you should avoid subscriptions where you can, because there is a real risk that they'll change the rules to take away what you paid for down the line.
And especially don't trust the advertising company to not put ads in their paid tiers down the line.
And before you raise the hypothetical question of additional tiers in future, by that kind of logic people shouldn't make any recurring purchase of anything since in future terms might change.