Re: sounds like
Oracle usually shits a brick well before that mark. Depends in your query rate and how much horribly written PL/SQL you're throwing at your doddering old mainframes. I've lost count of the number of places that have come to us wanting to replace their Oracle boxes, thinking they'd pick up something like Teradata or Vertica, only to end up settling for Hadoop.
Between the completely hassle free configuration and administration, the resilience and fault tolerance, the as-good-as-free hardware and licensing and the massive toolkit you get, it's pretty much a no brainer. Especially now the major distributions come with their MPP SQL engines (Impala, Stinger etc.). Throw in Parquet and Snappy and you literally have a carbon copy of your garden variety Teradata architecture, but open source and really, really sodding cheap.
Those in traditional data warehousing should be worried.