The foundational problem with interoperability is that it can and will immediately be abused by bad actors as long as there is no price tag attached to every piece of communication.

Among social media, Mastodon (and anything Fediverse) has it the worst, obviously, but Telegram and Whatsapp are rife with spams and scams, Twitter back when it still had third-party apps was rife with credential and token compromises (mostly used to shill cryptocurrencies).

As for the price tag reference - we've seen that with SMS. It used to be the case that sending SMS cost real money, something like 20 ct/message. It was prohibitively expensive to run SMS campaigns. But nowadays? It's effectively free at scale if you go the legit route and practically free if you manage to get someone's account at one of the tons of bulk SMS providers compromised. Apple's iMessage similarly makes bad actors pay a lot, because access to it is tied to a legitimate or stolen Apple product serial.

Paywalls can have the opposite of the effect you want. Implemented incautiously, they can fail to disincentivize parties who can make profit in excess of the cost, and it can succeed at disincentivizing genuine, non-profit-motivated interaction.

Imagine how much less you would use text messages if they still had a per-message cost.

Because some hostile entity might rat fuck the a slightly better system, we're destined to use the same current shitty system because something better might have a downside?

Do you understand that this is all literally made up? The rules can change anytime and society can exert its will to make better world rather than letting a dozen people decide how technology will shape humanity (mostly in a negative capacity if you look at the current state of things).

But bad actors already do this, as there is a monetary incentive to implement adversarial interoperability. There is then an incentive to not scale it up too much, lest that implementation get cut off sooner. For example, I certainly don't think all of the spam ads I see on Faceboot Marketplace are from individual people manually creating accounts and typing them out.

This is a confusing comment. Interoperability and bad actors are separate concerns, because you get bad actors in systems of all kinds, not just in interoperable systems. Paywalling a system does not necessarily mitigate bad actors, either.