More or less agree with Snark Knight, except I would extend his reasoning to say that whatever "runoff" happens from D&D to other games happens on a delay. This is a natural outgrowth of playing the game. If 1,000 people in your area start D&D 5E tomorrow, then some subset will stick with it--say 500 for sake of example. Out of those 500, some much smaller group will enjoy the idea of roleplaying but will eventually become dissatisfied with how 5E handles things. A subset of those will go looking for something else. Some will try other games but not find one they like more than D&D. Others will not be able to talk a group into playing another game. But there are two key aspects of how this works:
1. The GM's are a huge driver. If I keep running 5E games, 20+ odd people are going to keep playing it. If I switch, most or all of them will switch with me. Out of those players, at least three are also involved in other groups with a different GM. If they like what I switch to enough, it may cascade. But probably will not.
2. Dissatisfaction grows at a different rate for different players and there are a variety of reasons for it. The switch tends to happen when a majority of the group are dissatisfied with the same aspects (or mostly the same). Otherwise, you get the "everyone's second choice" thing, which is not merely 5E versus other versions of D&D but also 5E versus some other game.
You also can't discount how much of a deal-breaker the particular dissatisfaction is. If something just annoys you, but you like the group a lot, and none of them are bothered by the thing you don't like, then you'll probably put up with it. The severity of the dissatisfaction has to rise to the point that it overcomes inertia.
Bottom line: I think it is only recently that 5E has been around enough that we will start to see such migration, if we do.