Still with Cassandra you loose trnsactions and you can't start "small", as small as you could start with postgresql. A 10000 user service works well with a database running on 512mb ram, while cassandra needs at least 1gig (running lower could work there also, however mostly you will run into longer gc pauses than, it's really hard to trim the jvm down to less. I mean there are some ways to do it but it's way harder than just using something different).

and mostly people start with as low as 10.000 people. Scaling means starting from 1 going to X. and postgresql is well enough for most stuff. You don't need multi-master replication for most stuff. people mostly don't need write scalability. and for stuff that needs some writes they could easily cache that. (see instagram)