Spark may be a mature solution for truly big data, in a SQL like fashion, 1TB and more. But I constantly see it being misused, even with datasets as small as 5GB. Maybe the valuation of the company reflects this 'growth' and 'adoption'. And data locality is a thing. You can't read terabytes from object storage (over http). The batch oriented, map reduce is not going to be conducive to too many ML algorithms where state needs to be passed around.