The danger when a majority of users is relying on SPV wallets (by belcher_)

This is a repost of a post by u/belcher_ on the bitcoin-dev mailinglist (source). I think it is a good description of why relying on SPV wallets (or someone else's node) can be dangerous for bitcoin from a "macro perspective. For clarification: SPV means "Simplified Payment Verification". In very simple words, it's a wallet that doesn't verify everything on the network (as opposed to what a full node does), and can be fooled in certain attack scenarios, as described below, f.ex.For example, consider if bitcoin had 10000 users. 10 of them use a full node wallet while the other 9990 use an SPV wallet. If a miner attacked the system by printing infinite bitcoins and spending coins without a valid signature, then the 9990 SPV wallets would accept those fake coins as payment, and trade the coins amongst themselves. After a time those coins would likely be the ancestors of most active coins in the 9990-SPV-wallet ecosystem. Bitcoin would split into two currencies: full-node-coin and SPV-coin.Now the fraud miners may become well known, perhaps being published on bitcoin news portals, but the 9990-SPV-wallet ecosystem has a strong incentive to be against any rollback. Their recent transactions would disappear and they'd lose money. They would argue that they've already been using the coin for a while, and it works perfectly fine, and anyway a coin that can be spent in 9990 places is more useful than one that can be spent in just 10 places. The SPV-wallet community might even decide to use something like invalidateblock to make sure their SPV-coin doesn't get reorg'd out of existence. There'd also likely be a social attack, with every bitcoin community portal being flooded with bots and shills advocating the merits of SPV-coin. This is not a hypothetical because we already saw the same thing during the scalability conflict 2015-2017.Before you know it, "Bitcoin" would become SPV-coin with inflation and arbitrary seizure. Any normal user could download software called "Bitcoin wallet" which they trust and have used before, but instead of using Bitcoin they'd be using SPV-coin. You may be one of the 10 wallets backed by a full node, but that won't do much good to you when 9990 users happily use another coin as their medium of exchange. via /r/Bitcoin https://bit.ly/3f1AHrU