This limitation of Pure's design is it builds off of the midrange, modular, scale-up, failover controller design which has dominated midrange storage systems for two decades.
This "vertical" design makes sense when the bottleneck is the disk drives, controllers represent a significant part of the cost, and scaling over time is accomplished by adding disk trays. But will flash, the controller is the bottleneck, and the flash media represents the more significant part of the cost.
Scale out, or "horizontal" designs make more sense with all-flash. XtremIO has an interesting scale-out design, where essentially each disk tray has a dedicated pair of controllers. Solidfire also has an interesting scale-out design, but does not have parity RAID, which, given the high per-GB cost of flash media, creates challenges. Kaminario may have the most interesting scalability story.
If Pure can build a vertical and horizontal scaling design similar to Kaminario, that would significantly enhance their product.