They already do this [1]. They used to sell this access to 'value-add' data analytics resellers as well, but in 2015 they acquired one instead [2] and stopped reselling it [3].

[1] https://dev.twitter.com/streaming/firehose [2] https://blog.gnip.com/twitter-data-ecosystem/ [3] http://thenextweb.com/dd/2015/04/11/twitter-cuts-off-firehos...