I just recalled one of the only funny gags from Airplane II, which was the proposed movie advertising poster for "Rocky XXXVIII", showing a decrepit old man in boxing gloves. (Not so far from the truth as it turned out.)
In 90 years will Star Wars suffer the fate of the Super Bowl when they have to resort to "Episode L" after all of those might "X" years?
You know, @Johnny_Larue, you might just have hit the mark: Lengthy Roman numerals do look a bit ridiculous on a movie poster, so that might be why they don't call "Star Wars 8" "Star Wars VIII".
Also, they would have to use the numeral IX for the ninth movie. I'm not sure whether a large part of the audience would understand how Roman numerals work and what it means.
Frankly, if you don't know how to read Roman numerals by the time you're in your teens, consider yourself a moron.
Well, you have to consider that these movies are made for an international market, too. I don't know whether Roman numerals are part of the eductional curriculum in, say, China or Thailand. Besides, isn't Star Wars for morons, too?