Yup, I agree with you. Most businesses can work fine with traditional RDBMS, even on TB scale if you have a large box with lots of RAM and SSDs.
My example was just one use-case where one should consider using a more unconventional DBs like Cassandra or HBase.
In most cases, you have colossally fewer than 1M writes per second, and you usually distribute them over several boxes. the IoT avalanche of writes in particular is naturally sharded, that is, all these writes can occur independently, and then you can replicate statistics into an analytical RDBMS.
It's a large write load that needs to be transactionally ordered, like a stream of purchases in a large online store, that can be a hard(er) problem for an RDBMS.