If only it was true! Now we're almost four years after the release of John Carter (it had been released in the US on March the 9th of 2012), and there are still regularly articles hinting at the possibility of sequels to the movie. Here is one linked, from January the 8th of 2016, on the moviepilot.com site, by Adonis Gonzalez. The author writes about those franchises we thought were dead, and which come back in some way or another. On these 5 chosen, 4 had their sequels greenlit: Star Wars, Harry Potter (sort of, with David Yates' Fantastic Beasts...), Avatar, Pirates of the Carribeans, and... in 1st place, John Carter! He states the October 2014 information which said that Disney gave back the movie rights to ERB Incorporated, and that ERB Inc was seeking new partners for a followup, until now officially to no avail.
In any case, here is another proof of the ever-growing popularity of the movie. I still find on a daily basis tweets posted with the #JohnCarter hashtag, from people that are still discovering and loving the movie. The audience is right there, right now, and if Disney is smart enough, they should produce the seuqle right now!