What you're describing here is Roblox (2006), which was launched 2 years before Bitcoin was conceived, and has attained a market cap of >$50 billion without any cryptocurrency nonsense. Or Second Life (2003). Or RuneScape (2001).
Cryptocurrency doesn't enable online trading with virtual currency -- if anything, it's dramatically _more_ difficult to use than either real plastic, or virtual currency sitting in a SQL table.
What it enables is theoretical trust in a shared ledger (handwaving that a Sybil attack isn't attempted by any of the massive mining pools); but this trust is assuming you're a computer scientist capable of auditing the clients and contracts. Yet, even professional programmers have lost hundreds of millions of dollars worth of crypto with programming mistakes and security flaws in their smart contracts and clients! If even professional crypto software developers can't take advantage of cryptocurrency's supposed benefits, how are the players of an online game supposed to?
The answer is of course that they have to relegate trust, in the same way that online shoppers relegate trust to Visa/Mastercard. The important difference being that only Visa/Mastercard guarantee your funds are returned if you're defrauded.
Sybil attacks would be detected in an open network first of all. If you can pull it off, I invite you to try: there's hundreds of billions of dollars on the table protected by nothing but cryptography and game theory. Knock yourself out. I'm sure all the people in this space are idiots and haven't seen the holes you claim are there.
I'm old enough to remember when using a credit card was a hassle or seen as predatory on consumers and many people either refused to use credit or accept credit in their businesses. For several decades using a credit card wasn't any more convenient than using cash or check even. Over time, the scanners got better and faster, Visa and MasterCard integrated with ATM debit cards, and then mobile phones and swiping plugins enabled it all to get better. Now it's ubiquitous. But that ubiquity will make it all the more easier for merchants to adopt crypto.
It turns out there is already crypto insurance protocols like Nexus Mutual and Cover that will cover you if a loss occurs. Insurance is really just a business problem that is easily fixable.
On the plus side crypto is far more secure than Visa or MasterCard, because it works on public/private key cryptography and you simply sign a transaction rather than enter your name and address and all the same information you'd need to commit a fraudulent credit card transaction.
The point is that no-one needs to adopt crypto for payments because the existing payment platforms work fine, better than crypto. Making transactions immutable, requiring users to maintain perfect IT security, currencies swinging in value by 20% every time Musk tweets about them, etc, are regressions.
Also for your other points - the games in this space are universally unfun to play with hardly anyone actually playing them; I can pay normal money to commission artwork; and I can use a zillion platforms that already exist to raise money.
Second Life was awesome! This Metaverse trend is pushing that concept much, much further and it's giving ownership and utility to the makers.
>you can buy a digital artwork at a Christie's auction and display it in Decentraland and have it be verifiably authentic
I can't imagine anyone would give a shit about someone's virtual art being "authentic" for whatever meaning of authentic NFTs pretend to claim. This is basically receipt porn at this point. Truly we're talking about bragging about wealth, one of the shittiest traits a person can have.
From a competitive standpoint, why would I want Activision's digital locks (you can only import NFT'd content) instead of something like Second Life which is more open?
If Activision really wanted purchases to carry over, they could've made it happen. No NFT needed. An open standard does not imply NFT, and vice versa. The 360 era was probably the peak of the 'gaming interoperability' trend, and it went almost nowhere. Your Xbox gamer picture would show up in some games. Everyone knew it was authentic because MS uses cryptography and doesn't let you sideload third party images. No NFT needed.
Same reason my company doesn't like me displaying awards from my last company. The obstacle is entirely social rather than technical.
This all happens in Second Life and doesn't need to burn the planet to do it.
Anyway, none of this matters because Bitcoin exists and is the dominant shitcoin (due to first mover advantage). All these other shitcoins can claim whatever efficiency, but it doesn't matter because that's a very isolated view of the problem. The shitcoin economy as a whole will still be fueled by mining unless somehow Bitcoin gets dethroned and the whole mining thing just goes away. I can't imagine that happening without the whole economy crashing completely into the ground – but that would bring all these efficient coins down with it.
And already more transactions and more daily volume is settled on Ethereum than Bitcoin, see https://money-movers.info/ and https://cryptofees.info/ (pink lines are Ethereum DeFi applications)
https://www.sandbox.game/en/ https://decentraland.org/
These are 3D virtual world environments and people are using crypto to build an online economy. They are trading property, buying virtual artwork and creating galleries, setting up clubs and meeting spaces, etc.
You can also explore decentralized finance, act as a peer-to-peer lender, provide liquidity in a marketplace, fund an idea you're interested in, whether that's a game (https://illuvium.io) or a commission for a comic book series (https://www.one37pm.com/nft/art/punks-comic-pixel-vault-cryp...) or start a DAO and raise money for charity or open source software (https://gitcoin.co/).