Recently, there was an interesting poll on the Necropraxis blog about D&D being a gateway game for table top role-playing games. I think we have to be careful about assigning much significance to it. The people who are interested enough in Dungeons and Dragons or table top RPG’s to be following Twitter accounts, blogs or Reddit feeds on the subject are a small fraction of the people who actually play. The bulk of the people engaged in online interaction are, I believe, primarily game masters or people who want to become game masters. I saw a poll in a Basic/Expert D&D Facebook group recently, it turned out that almost everyone in the group was a DM. Not scientific, I know, but it is a bit of anecdotal evidence that suggests we have to be cautious about the conclusions we draw from social media and online polls.
The real question, which is impossible to answer: How many people who have played D&D go on to play another TTRPG at some point?
My suspicion is very few.
Rather than thinking of D&D as a gateway to the TTRPG hobby, it may be more useful to think of D&D as it’s own hobby. Yes, it is a table top RPG. Yes, many people who go on to play other games start with D&D but very few who start with D&D go on to play other games. Even fewer abandon D&D altogether to play other games. Many people who play Shadowrun played Dungeons and Dragons first but very few people who have played Dungeons and Dragons will ever play Shadowrun. It is extremely unlikely that someone who started with Shadowrun will never play Dungeons and Dragons. I’m sure it happens but it is not frequent that someone who is an RPG hobbyist never gets around to playing D&D.
TTRPG’s require a significant input of time and effort on the part of the game master. For a game master to pick up a new game, read through the material and learn the rule set well enough to be able to run even a single entertaining session is an investment of time and effort. At some point in this process the GM has to find players willing to try the new system. Even for 5E, it requires some effort to assemble a group of players who can coordinate their schedules well enough to put together a regular game. Finding a group willing to try a game they’ve never heard of… even harder.
The outcome is that for a lot of players, D&D is the hobby.
They’ve never played another TTRPG and they never will.