Privacy from whom though? I’d contend that more than 99% of people reading this would, in all honesty, consider their adversaries to be non-state actors—marketers, hackers and identity thieves. For the less than 1% of us for whom their adversary is a state actor, I would contend that any implementation of SMTP is non-viable, period.
If you’re sufficiently paranoid, the only thing you can say for certain is that there isn’t much you can truly trust.
This is the key. Model your threat and act accordingly.
There's (mostly) no reason for Joe Bloggs to want to protect themselves from a well funded state actor but they may want to prevent themselves being locked out of Google services because some bot triggered a threshold.
If you’re sufficiently paranoid, the only thing you can say for certain is that there isn’t much you can truly trust.