Monopoly money does prevent spam. In decentralized networks preventing people from DDOSing the network becomes extremely important and very difficult.
Spammers have money. For them it's merely a cost of doing business and they'll spam as long as there's positive ROI. For something like politically-motivated disinfo on Twitter there are millions of dollars bankrolled.
OTOH any non-zero cost is off-putting for regular users and stifles growth of services. Twitter has upset its top users by trying to change $8!
Then there are flash loans that let you have any amount of wealth in the web3 world for only a tiny fraction of the price. It keeps popping "DeFi" services that naively assumed nobody would risk a fortune to attack them, but there's a fortune-for-hire service!
Anyway, the real-world web has used phone numbers as proof of small amount of money. There are CC checks for bigger amounts. Then there's web of trust which is a pre-web3 word for "staking", except using reputation instead of Monopoly money. Mastodon instances for example ban other instances from federation if they don't police spam/abuse enough, so every server admin is motivated to fight it locally, while the system as a whole remains decentralised.