Also, if someone can break sha256, not only is bitcoin screwed, but so is the Internet as we know it and every secure institution period. The overnight "instant and complete" breakage of fundamental assumptions about security would destroy everything. So interestingly, if you could break sha256, you could probably break into almost any financial institution and steal their money as well.
The security of a blockchain is in its popularity. As long as the code is open (and I'll admit, I am not a fan of how bitcoin mixes the default interface (BitcoinQt) and the default implementation (BitcoinCore) together so much) and there is investment in the market, there will be strong financial pressure to consistently and diligently analyze and track the source code for weaknesses.
Bitcoin at 3B has at least a dozen security firms that have looked into its crypto. If you had another crypto that broke a trillion you would have thousands of companies constantly safeguarding and testing the crypto of the currency in the same way Google offers ludicrously high bounties for anyone who can break Android/Chrome/ium security.
I don't necessarily blame Ethereum for this, although they did have a hand in following the banks' rules and essentially writing software for them, but this would've probably happened even without Ethereum agreeing to do it. Maybe the banks would've forked Ethereum's technology and then they could've done whatever to it, or they could've written their own tech from scratch.
That will eventually be fixed, and once we are back to normalcy where transactions aren't limited by block size, you can regularly get - even on feeless transactions - multiple confirms in seconds. Your transaction won't be finalized until it enters the block, but once you have a half dozen or more confirms the probability of failure of that transaction entering is extremely low - well below 1% at that point, insofar as I have never seen it happen myself and don't know anybody who has had a multi-confirm transaction miss acceptance.
The self-checkout technology will never improve as long as that assumption remains. If you can't trust the customer to not steal a pack of gum, you can't trust the customer to not steal an entire beef tenderloin.
The professional cashier does not need to keep every damned item on a giant scale after scanning it, because they are an agent of the store, and presumably trustworthy enough to notice and stop shoplifters and scammers. And they usually don't have to halt the entire checkout process and wait for a manager to come by and restart it whenever anything even mildly out of the ordinary happens, such as using a coupon.
You can't improve the technology when the premises are wrong. And the premise is that you can turn a grocery store into a glorified vending machine.
Remember, the store is more interested in protecting its operating margins than in providing more convenience for the customer. If the technology does not allow a store exec to believe that it can stop a $30 steak from walking out the door unpaid, it will never be installed.
- The current biggest problem that bitcoin faces is one of scale - and there's no clear solution in sight. Why on earth would you want every little transaction on the blockchain? If I buy a stick of gum, that doesn't need to be on public record for eternity. It is impractical and unwieldy.
- Centralized systems are always more efficient than decentralized ones, and systems naturally tend toward centralization. Bitcoin falls down under centralization, which we are also currently witnessing with the hash power tied up by a few largely unknown Chinese operators.
There are proposed solutions like pseudo-centralization with LN where bitcoin just becomes a settlement layer for off-chain contracts - and the details behind LN are still so primitive it is not clear it will actually solve any real problems.
What you are talking about in practice for something at scale is a centralized blockchain of some sort, which is really just a distributed, immutable database, with some sort of trusted signing authority. What exact advantage does that give aside from potentially non-reversible transactions, which are a clear non-starter for most consumer purchases? There are already services that offer true non-reversibility for special cases where this is desired.
There has so far been no viable application for blockchain technology as a replacement for cash except black/grey market purchases.
Further, even with a total secure system, there will always be the pressure of the governments to control it.