"Avoiding disruption is crucial for the three incumbents, because they want to keep and extend their business with database customers."
This is the key. Certainly all the macro trends are deeply disruptive. Fast mobile networks, broadly deployed sensors, real time electric grids ... all of the components of "Internet of Things," "Machine to Machine," and "Smart Planet" initiatives that are producing data that requires fast, real time processing at scales not possible in legacy database systems.
This disruption is driving multiple avenues of change in the in-memory database space. Most interesting to us, at VoltDB, is the use of in-memory decisioning and analytics to ingest, analyze, organize, respond to and archive incoming high velocity inputs. Combining analytics with transactional decision making on a per-event basis is necessary in a large class of use cases: real time alerting, alarming, authorization, complex policy enforcement, fraud, security and DDoS attack detection, micro-personalization and real time user segmentation.
Viable solutions must be virtualizable for cloud deployment, must offer integrated HA, must run on commodity cloud servers. Adding in-memory without eliminating the expense and complexity of shared storage is insufficient.