If setting it to 3 becomes standard, how do new identities gain sufficient trust to be visible to the people they need to trust them? If any positive number is chosen, new users are isolated and struggle to build trust. If zero or only slightly below, only a few blocks could sink them, and that could be trivial to achieve on simple polite difference of opinion. If set significantly below zero, we're effectively setting a maximum number of people you're allowed to piss off before we drop you and you just... Make a new npubs. None of these scenarios can simultaneously welcome new users AND provide a meaningful obstacle for spammers.
PoW isn't my favorite option, but I think there would be a large difference between enough difficulty to discourage wide spread spamming and enough to be of issue to actual users. 5-10 seconds of PoW in the background wouldn't really impact your battery life unless you were sending dozens, maybe hundreds, of events a day, but it means a spammer on a typical user device is limited to 6-12 spam messages a minute, which may add up to 8,640-17,280 messages a day, but spammers often rely on orders of magnitude more messages due to the terrible conversion rates for spam. It also means the spammer has to dedicate an average device to that full time, all day, to achieve that, increasing the hardware and electricity cost to do it. As an added bonus, bots that don't provide a meaningful service the community is willing to support now have a financial cost attached, discouraging them from polluting the network with well meaning but unappreciated bulk messages.
