containers != microservices
Certainly not what Netflix are describing in any case.
Containerisation is a well understood technology that means a particular thing.
Microservices, far less so. It's mostly used as a synonym for application distribution. You may, or may not, put the distributed components in containers for ease of deploy, management, whatever.
Containers though would put absolutely no load onto cassandra, its the app inside that does.
If you are going to point out immaturity in an area, it's the current commercial understanding of distributed computing, which is fairly awful. This leads to, for example, something I like to call 'inner join over http', a common enough issue in these kind of developments, caused by the incorrect separation of data by entity, rather than by access pattern and something that is quite likely to be the root cause of the issues described.
Also worth pointing out that Netflix only adopted containerisation recently, previously they were on AWS EC2 exclusively.