"If you're referring to structured records, I saw the mainframe, I used the mainframe, and I was unimpressed."
You saw a mainframe. I saw a number that were quite different from each other. The parent said "a step back," though, not mainframes or a specific mainframe. There were many architectures that came before or after UNIX with better attributes as I list here:
https://news.ycombinator.com/item?id=10957020
If we're talking minimal hardware, let's look at two other approaches. One was Wirth's. They do an idealized assembly language to smooth over hardware or portability issues. It's very fast due to being close to bare-metal. Simple so amateurs can implement it. They design a safer, system language that's consistent, easy to compile, type-checks interfaces, can insert eg bounds-checks, and compiles to fast code. They write whole system in that. Various functions are modules that directly call other modules. High-level language, rapid compilation, and low debugging means that two people crank out whole system & tooling in about 2 years. Undergrads repeatedly extend or improve it, including ISA ports, in 6mo-2yr per person. A2 Bluebottle runs insanely fast on my 8-year-old hardware despite little optimization and OS running in a garbage-collected language. Brinch Hansen et al did something similar in parallel on Solo OS except he eliminated data races at compile time with his Concurrent Pascal. Later did a Wirth-style system on PDP-11 with similar benefits called Edison.
On functional end, various parties created the ultimate, hacker language in LISP. Important properties were easy DSL creation, incremental compilation of individual functions, live updates, ability to simulate any development paradigm, memory safety, and higher-level in general. The LISP machines implemented most of their OS's and IDE's in these languages. Imagine REPL-style coding of an application that would run very fast whose exceptions, even at IDE or OS level, could be caught, analyzed at source form, and patched while it was running. Holy. Shit. They targeted large machines but Chez Scheme (8-bit) and PreScheme (C competitor) showed many benefits could be had by small machines. Jonathan Rees even made a capability-secure version of Scheme which, combined with language safety benefits, made it one of most powerful for reliability or security via isolation. A project to combine the three concepts could have amazing potential.
So, yeah, UNIX/C was a huge step back in compiler speed/consistency, speed/safety tradeoffs in production, flexibility for maintenance, integration, debugging, reliability, security, and so on. Tons of architectures or languages better on each of these with some having easier programming models. That Thompson and Ritchie's perfect set of language features for C replacement were collectively an Oberon-2 clone (Go) is also an implicit endorsement of competing system. Plenty of nails in the coffin. Sociology, economics, and luck are reasons driving it. The tech is horrible.