What's less miserable and safer? What prevents not just message privacy but good metadata resistance? What prevents mapping out your social network? What pr

What's less miserable and safer? What prevents not just message privacy but good metadata resistance? What prevents mapping out your social network? What prevents an attacker from create 100M accounts and flooding all current users with invites and/or messages? I get that tying signal to a sim is less than perfect, but what is better?