We had this discussion around "the simplest thing" way back in the late 90s/early 2000s when XP was getting started.
One comment I remember well is, "it's do the simplest thing, not the stupidest thing".
Maybe a boring relational database is the simple thing,
compared to some distributed eventually consistent map-reduce system using HDFS.
Or maybe a simple key-value store like Memcached is all you need.
Even in complex domains, the aphorism, "everything should be made as simple as possible, but not simpler" applies.
As for working at scale, complex systems are made of small, simple, working systems, where dependencies between them are limited and manageable. Without an effort to keep interdependencies simple, the result is a Big Ball of Mud, and we all know how well that works.