I suppose in this scenario (I'm not saying it will or should happen btw) the article the person would have read would also mention this invite codes mechanism. Not sure how that would work but um yeah lol. They would still need to go somewhere for an invite code. Or use a pow service like special purpose version of a pow posting service like getwired.app to ask for one?
¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯ 

 I feel like I'm digging a hole here lol 

 i think pow is easy to implement without significantly affecting UX.
if default difficulty is something like 10, no users are gonna notice, but spam reply bots suddenly require a lot more resources 

 yea that was also my thinking, PoW won't help for one-to-many spam (as the work can be amortized over many targeted users), but for mass DM/reply spam it might, a bit
 

 yes, and one-to-many spam is where a ‘web of trust’ can be applied without any onboarding trade-offs