On the other hand, one modern server is 10,000x faster than a 1990s server with 10,000x the capacity, and modern RDBMSs are better at executing queries and give you more options for index types, so the 1990s single-server approach goes very, very far.

I suppose that over time we'll also see more data sovereignty laws, which gives a pretty natural partition key (i.e. for most data you needn't have a global serial because putting them in one database where you'd be worried about conflicts would be illegal anyway, and if you do have some place where you can do those joins, you can adjoin a partition id).