There are people who believe that proof-of-work isn't very effective, but none of them have succeeded in spamming the Bitcoin network with blocks they've mined, driving the other miners out of business, nor (for the last several years) with spamming the Bitcoin network with dust transactions they've signed, so I don't think we should take their opinions very seriously.
Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.
I don't see why invite codes would be a problem for mainstream use.
Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.
I don't see why invite codes would be a problem for mainstream use.