On other side AWS now offers 2TB RAM machine. And single huge machine has smaller per GB cost than several smaller machines. I think clustered computing as we know will be soon gone. Only reason for multiple machines will be availability.
And for that matter, even when we can't stuff it in RAM, the boundaries of what we can do on a single server is also constantly pushed back thanks to SSDs. It's just a few years ago since I was unable to get read speeds of more than 6GB/sec out of a RAM disk. Today I have servers that easily do 2GB/sec out of NVMe SSDs.
It's not that we never need to go beyond a single server. But people often really have no concept of when they'll need to.
VISA does what, 150M transactions per day? I was doing more volume than that for telephone calls a couple years ago with a $5K server and a default install of MSSQL. (Full ACID, updating balances -- yes I know a CC tx is probably heavier than a VoIP call but still.) At 4KB per tx, VISA could use VoltDB and store all tx's in RAM for a week for like a million or two.
An 150M a day today is not that much more than it was 15 years ago it seems. (50-100% more?).
For many, many, businesses, data size just isn't an issue any more cost wise, and soon won't be a technical challenge at all either. Yet 10, 20 years ago, we're talking 8-digit+ implementations.
Sure there's some things that grow faster, like all this increased tracking. But in general?