TripIt was acquired by Concur (who are in turn owned by SAP) many, many years ago - which is a move that clearly wasn't done in order to benefit from TripIt Pro's subscriber income.

The expectation at the time was that they would integrate it into their other products in some sense or other, but this really doesn't appear to have happened.

If TripIt dies it'll be due to one of those "we decided to go in a different direction" things rather than due to a lack of Pro subscribers.

Interestingly, Concur's website implies that Concur users can get complimentary TripIt Pro - but when I try it I get an error that I don't have access to that page (after logging in to Concur).