Your data is the most valuable thing you have to offer and you give it up for free. Why would they install a barrier to people giving away the the most monetized asset in their business sector?

Why would they let you avoid seeing ads? A user who doesn’t look at ads to drive the value of user data is worse than a non-user; at least a non-user doesn’t cost them any overhead.

How much would you be willing to pay? $10/mo, $50/mo ?

They see more potential in keeping it free and selling you then trying to collect money from you.

It would only be a matter of time before they started showing ads to paying users too. Pay for a magazine subscription? Magazine is filled with ads anyway. Pay for a cable tv subscription? Filled with ads anyway. Pay for HBO? Get shameless product placement (they claim they don't take money for it, but that doesn't change the substance of it: https://www.youtube.com/watch?v=TuIJVLrIAZQ) Pay for netflix? More product placement. Pay to see a movie? Ads before the movie, product placement throughout. Pay for Hulu? More like paying for ads.

Yes, but you are purchasing the magazine, movie, or service. If you are paying Facebook specifically to remove ads from an otherwise free product, it's much harder to justify injecting ads anyways.

And if for whatever reason they do, you can just stop paying them.

That is why I got rid of my tv antenna and pay for cable television, so I don’t see any ads.

People with disposable income to spend on web subscriptions are exactly who all the advertisers want to advertise to!

They have a paid option. Advertisers pay them to show you ads and get your data.

You are not a customer. You are the product. Produce doesn't get to chose how and where it is displayed.

This!