The reason disk never replaced tape was due to the high ongoing cost to keep it alive in a datacenter. Disks require a lot of power, cooling, and rack space, and their footprint only grows over time, all while dealing with failures of the mechanical bits which drive up operational costs to replace them continually. Flash technology doesn't have these same limitations. Flash will absolutely replace disk and tape.
For those that don't believe this consider it took 50 years for the HDD to get to 1TB. Samsung is shipping an 8TB SSD later this year. In order for the HDD manufacturers to increase drive density they need to retrofit their assembly line for HAMR, and even that technology hasn't won out yet. HGST was doing helium filled, SMR is here, and even bit patterned media is being toyed with. Either way, the cost is substantial to retrofit, and the big HDD manufacturers have acquired NAND media makers over the past few years. The writing is on the wall. Invest in HDD tech, or invest in flash? The market is speaking now, and if you listen, you will hear it too; disk is dead.