Why are you so eager to give up interoperability when it’s not the problem? Spam exists within walled gardens too. Interoperability and spam are orthogonal.
If the only way to preserve interoperability is to accept decades of unresolved abuse and perpetual patchwork fixes, then that’s not a trade-off I find compelling. At that point we’re not debating facts, we’re debating tolerance levels — and mine is lower. I think that’s a good place to leave it.