Ask HN: Why hasn't the cloud killed the mainframe?
First of all, I find I kinda funny that you call banking, retail and insurance &quot;legacy industries&quot;.<p>I would rather be without Netflix and Google, than banking and food ... but each to their own..<p>While some is inertia (mostly doing to rewriting truly large applications are hard and expensive), there is also the the point that most of those industries cannot easily handle &quot;eventually consistent&quot; data..<p>Not all transactions are created equally, the hardest usually have a set of requirements called ACID.<p>ACID in the classic RDBMS is not a random choice, but driven by real requirements of their users (the database user, i.e. applications in the business sense - and not the users as people). 
The ACID properties are REALLY hard to do in scale in a distributed system with high throughput. Think of the rate of transactions in the bitcoin system (500k&#x2F;day with many, many &quot;servers&quot;) vs. visa (500M+&#x2F;day) - the latter is basically driven by two (!) large mainframes (with 50ish km distance) the last I heard of any technical details.<p>None of the companies you mention need to have strict ACID, as nobody will complain if different users see slightly different truths - hence scaling writes is faily easy.