Sharding is, of course, trivial on those systems - after all, they're extremely feature-poor, and have given up those features specifically to support trivial s

Sharding is, of course, trivial on those systems - after all, they're extremely feature-poor, and have given up those features specifically to support trivial sharding. You can replicate much of this behaviour using open source RDBMSs, but yeah, it's not what they're designed for and it's harder. If you want quality replication/clustering you're currently looking at paid-for DBs. Being able to scale out is absolutely a nice-to-have. I'm not sure it's a nice-to-have on the scale of giving up all of the features an RDBMS provides for most people's use-cases. Further, you might find that the relative lack of data headaches you get with an RDBMS more than makes up for a little extra time setting up hot standby. Finally, if 100% uptime is that important you're probably not relying on a relatively niche NoSQL database. If uptime on the level of Stack Overflow is good enough (which for most people it probably is, let's face it), then you'll probably find replicated postgres good enough.