If I was Datastax I would be scared of Scylla. They have momentum and they are one of the best engineering teams around outside of top teams at Google/FB etc.
Maybe I'm too far out of the loop but while they both provide enterprise Cassandra they both also offer a bunch of different products so in some product categories there may be plenty of room for both of them. I gotta say though, going to each of their websites, and Scylla was very straight forward "here's what we provide" and I understand it pretty quickly. Datastax? I...still don't entirely understand what they off…
But last I heard they bought in the titandb guy to build their own enterprise graph database on top of cassandra.
So they got that going for them.