its not a drop in. There is no reasonable way of replacing a huge existing cassandra cluster with scylladb. You have to transition as it was a new database.
Why is java bad here...? Because of GC?
Directly and indirectly, there are a lot of other knock on effects. Value objects (only recently being added) help. You could treat it similar to an HFT application, having ephemeral workers that don't do GC and dies on heap exhaustion, with a NIO coordinator that tries to avoid all garbage creation. But why at this point, square peg round hole.
yeah, i suppose one could start with scylladb as their base today.