The fundamental security limits of bridges are actually a key reason why while I am optimistic about a multi-chain blockchain ecosystem (there really are a few separate communities with different values and it's better for them to live separately than all fight over influence on the same thing), I am pessimistic about cross-chain applications.

To understand why bridges have these limitations, we need to look at how various combinations of blockchains and bridging survive 51% attacks. Many people have the mentality that "if a blockchain gets 51% attacked, everything breaks, and so we need to put all our force on preventing a 51% attack from ever happening even once". I really disagree with this style of thinking; in fact, blockchains maintain many of their guarantees even after a 51% attack, and it's really important to preserve these guarantees.

For example, suppose that you have 100 ETH on Ethereum, and Ethereum gets 51% attacked, so some transactions get censored and/or reverted. No matter what happens, you still have your 100 ETH. Even a 51% attacker cannot propose a block that takes away your ETH, because such a block would violate the protocol rules and so it would get rejected by the network. Even if 99% of the hashpower or stake wants to take away your ETH, everyone running a node would just follow the chain with the remaining 1%, because only its blocks follow the protocol rules. More generally, if you have an application on Ethereum, then a 51% attack could censor or revert it for some time, but what comes out at the end is a consistent state. If you had 100 ETH, but sold it for 320000 DAI on Uniswap, even if the blockchain gets attacked in some arbitrary crazy way, at the end of the day you still have a sensible outcome - either you keep your 100 ETH or you get your 320000 DAI. The outcome where you get neither (or, for that matter, both) violates protocol rules and so would not get accepted.

The fundamental security limits of bridges are actually a key reason why while I am optimistic about a multi-chain blockchain ecosystem (there really are a few separate communities with different values and it's better for them to live separately than all fight over influence on the same thing), I am pessimistic about cross-chain applications.

To understand why bridges have these limitations, we need to look at how various combinations of blockchains and bridging survive 51% attacks. Many people have the mentality that "if a blockchain gets 51% attacked, everything breaks, and so we need to put all our force on preventing a 51% attack from ever happening even once". I really disagree with this style of thinking; in fact, blockchains maintain many of their guarantees even after a 51% attack, and it's really important to preserve these guarantees.

For example, suppose that you have 100 ETH on Ethereum, and Ethereum gets 51% attacked, so some transactions get censored and/or reverted. No matter what happens, you still have your 100 ETH. Even a 51% attacker cannot propose a block that takes away your ETH, because such a block would violate the protocol rules and so it would get rejected by the network. Even if 99% of the hashpower or stake wants to take away your ETH, everyone running a node would just follow the chain with the remaining 1%, because only its blocks follow the protocol rules. More generally, if you have an application on Ethereum, then a 51% attack could censor or revert it for some time, but what comes out at the end is a consistent state. If you had 100 ETH, but sold it for 320000 DAI on Uniswap, even if the blockchain gets attacked in some arbitrary crazy way, at the end of the day you still have a sensible outcome - either you keep your 100 ETH or you get your 320000 DAI. The outcome where you get neither (or, for that matter, both) violates protocol rules and so would not get accepted.