Like Joel Spolsky said it: "They followed the single worst strategic mistake that any software company can make: They decided to rewrite everything from scratch" - http://www.joelonsoftware.com/articles/fog0000000069.html I think rewriting from scratch is the core of their problem and not really Cassandra. Gradually going over to Cassandra would have been a much better idea.
Don't ever develop yourself into a one way street!!