I don't think it has anything whatsoever to do with JMS' insecurity (or lack of). The truth is film studios are interested in what is going to make them money. Pure and simple.
We can sit here and wax lyrical all we like about how a good B5 movie will blow away the competition in terms of story. characters, whatever, but (and here is the rub) ...
Where is the big headline movie star to "sell" the movie?
JMS has already said categorically that he sees a theatrical movie as a way of giving the regular cast a showcase (and big payday) as a thank you for their efforts on the series. None of the series regulars would sell a B5 movie to the general movie-going public.
What would it have that will make it a "Blockbuster"?
Like it or not, successful SF movies tend to be big budget, FX laden, "event" movies with less emphasis on a complex and coherent story or deep, well-rounded characters.
Don't get me wrong, I would love to see B5 on the big screen but then I have been hooked on the show since "Babylon Squared" (hey answered two threads with one post - cool). It is the casual SF movie-goer that has to be convinced if WB is going to make any money from a B5 movie.
That means a story that stands all by itself, without the need for any prior knowledge of the B5 universe, and JMS' record in producing one-shots unrelated to the main story have in the main been disappointing.
The best of the TV movies are (IMHO) ItB and ACtA, both of which are tied very closely in with the main story arcs of B5. By contrast Thirdspace, RoS, Rangers (so I'm told) all gave fans cause for concern (the sense of disappointment on the UK B5 usenet group when Thirdspace appeared was incredible).
The long-form story appears, to me, to suit JMS far better than one-shots, and given that we will soon be seeing SW:EpIII, the two Matrix sequels, LotR:II & III, Harry Potter II, III, IV, what odds on a JMS one-shot spin-off from a small cult TV show blowing the competition out of the water?
Not high I wouldn't think. Much as I hate to say that.
Anyway, enough ramblings from me, bottom line is ... if WB think it will turn in a healthy profit it will get made. Let's hope that's the case.
Cheers,