I would think that having Rifftrax or other riffer groups do the same movie would encourage sales, actually.
We pondered that very point for some time and decided that the best way to HELP it encourage sales would be to accuse the big dogs of stealing. In a humorous fashion, of course. All fun and games, we promise.
...Titanic is freaking long and has no cult appeal and therefore is unlikely going to draw in much of a crowd.
We also talked about this when choosing the vehicle for our first riff, and felt the cult appeal of Titanic is actually the very hatred the cult has for it.
I know you didn't ask for it...
We didn't, but we should have. You advice is informative, fair and even-handed.
It also doesn't hurt to grant the files to some forum members in exchange for ratings and comments as a promotional offer.
Also, you're a prophet. This was precisely our plan. Care to add yourself to the list? Message us.
Of course it also helps if you're funny.

Honestly, one of the bigger reasons we decided to do an "untapped" film was to stave off any comparisons to the jokes of pros with decades of experience. It's a bit intimidating when we have to go up against the guys who've been making us laugh since the fifth grade.
In all seriousness, though, it won't affect our riff one way or the other, we just wanted to declare that we weren't trying to ride on their coattails.
I know I only buy iRiffs when there's a regular Rifftrax of it too. I actually buy all the iRiffs that are copies of official riffs (or Rifftrax Presents), with the exception of that one fake iRiff for Red Dawn.
We promise no fake riffs. That's a Big Jim guarantee.
It is very encouraging to hear that there is no stigma amongst iRiffers against previously-plowed territory. We would be very sad, indeed, if we NEVER got to riff an Indiana Jones movie. Thanks for the kind words and thoughts.