A large part of the overhead of concurrent data structures lies not in the work itself, but in the synchronization used to avoid corruption to the data structure. The technique you're describing could potentially allow a resize to occur incrementally, without blocking other operations for a long time, but that doesn't take away the need to synchronize between resize, write, and read operations.

RCU-based algorithms allow readers to proceed with absolutely no locking, compare-and-swap, atomic operations, or other expensive steps. In order to support that, any modification to the data structure (such as a write or resize) must make sure the structure remains in a completely valid state after every individual modification. The hash resize algorithm I wrote provides that feature, ensuring that the resize does not disrupt concurrent readers.