IMO the biggest problem is that SQL joins don't scale on commodity hardware networks due to the CAP theorem, assuming you want availability. Network IO is a lot better these days, and per Scylla it can get to a pretty small instruction window, but still it isn't what XPoint and SSD crunch.

Database rearchitecture at the single node level can help optimize this, and maybe improve the megamachine SQL node to scale up to fatter sizes, but really big data is still a network IO and reliability challenge.

And it may be true we can pump a single fat node up to really huge throughput once some db rearchitecture fully leverages SSDs and I/O, but that just exposes more data to downtime if there is a network partition.