Blockchains are much more than receipt paper. Receipt paper is a superfluous byproduct. The blockchain is the system of record.
The biggest difficulty would be accurately identifying where the signature came from, but with clever timing mechanisms, it might be possible given the latency of light. I haven't thought about it much, but maybe require the current time be included in what gets hashed. Anyone cheating by using a future time could be caught by looking for missing transactions in that time space.
"The Casper protocol is intended to offer stronger finality guarantees than proof of work. First, there is a standard definition of “total economic finality”: it takes place when 2/3 of all validators make maximum-odds bets that a given block or state will be finalized. This condition offers very strong incentives for validators to never try to collude to revert the block: once validators make such maximum-odds bets, in any blockchain where that block or state is not present, the validators lose their entire deposits. As Vlad Zamfir put it, imagine a version of proof of work where if you participate in a 51% attack your mining hardware burns down."
https://blog.ethereum.org/2016/05/09/on-settlement-finality/
If a blockchain has rules that don't meet my needs, I can choose a different blockchain. If I adopt a blockchain that changes its rules to be undesirable, I can take the history I care about and move it to another blockchain.
This argument depends on proof-of-stake. Some people think proof-of-work blockchains are the only useful ones, but I disagree. It's also a boring debate to have, so I won't be doing that here.
Now, banks and such will be some of the biggest users; and your average Joe won't even understand what the problem is with trusting them with the blockchain. So... good luck with getting that consensus.
Had this happened with Bitcoin? I think I heard there are some quite major disagreements (something about block sizes and stuff) and there are a lot of users that aren't happy. I don't remember hearing them just "well, whatever" and forking off.
Ethereum is a general purpose blockchain. Forking it could hurt the value of ether, but for many applications, the value of ether doesn't matter. Moving a blockchain-based Twitter from one fork to another is relatively easy and consequence-free.
This is not true of most currencies. Most currencies are valuable because governments force you to pay them on threat of imprisonment. Bitcoin does not have this feature and therefore, yes, it's hard to see what its fundamental value is.
I'm not sure it's really consequence-free. It's essentially a netsplit, where the forking party lose connectivity with everyone who's left. Consider the case where Twitter (the centralized one) is suddenly falling apart over some issue into two distinct non-interoperating services. Technically - sure - it's a no-brainer, but socially it feels quite complicated to me.
(I guess, this would work if the app would be designed to use multiple blockchains. I need to give it some thought.)