Cortex author here (Tom Wilkie). Great post that honestly highlights the differences between these systems - thank you!
The biggest take home here - and the first thing the post mentions - is the a single HA pair of Prometheus servers is enough for 80-90% of people. TLDR you probably don’t need Cortex (or Thanos, etc)...
...unless you run multiple, segregated networks (regions). Then something like Thanos (or Cortex) is useful - not for a the scale argument, but because you need a way to “federate” queries and get that global view. IMO!