You're ignoring the real benefit of Drobo: mixing heterogeneous drives. Unfortunately, the cost of that feature is now really high, since it seems like regular RAID has gotten faster and cheaper while Drobo has stayed expensive and slow.
In the current market, Drobo has managed to price themselves OUT of that space while simultaneously NOT keeping up with performance on the higher end.
Drobo is now in the uncanny-valley of proprietary RAID systems.