I spent a lot of time with NoSQL systems in the early 2010s and I think what “killed” them was processors and networks getting so much faster that Postgres “just worked” for more and more use cases.
There are systems I’ve built in the past with 20+ Cassandra nodes and tens of thousands of ops that were originally built on MySQL/Postgresql but migrated to Cassandra because the performance/cost of the SQL systems was just to high.
Now those performance requirements can be handled cheaply with 1 or 2 beefy PostgreSQL databases. The level of scale you need today to make put up with something like Cassandra is much higher while yesteryear it felt like every startup was falling over once they found pmf