Pathfinder is an excellent game. It's also a niche product, which is why they are making their new releases (and retrofitting old ones) compatible with 5e, to tap into the exponentially larger D&D market. But D&D always cycles up and down. Typically, after each surge in popularity there is a tailing off as some of those new players get busy with life, bored with the game, or whatever, and others want to try new types of RPGs, such as Pathfinder for those who want an experience extremely similar to D&D but slightly more granular (for me, the Pathfinder vs. D&D debate is a lot like debating Coke vs. Pepsi; like, are these really meaningful differences?). This is healthy for RPGs.
OneD&D is WotC's attempt to mitigate that usual cycle by updating the game and offering new product without creating a wall for the existing player base. We'll see how it goes; I expect that this year will probably see slower growth in D&D and in the near future it will flatline or contract a bit until the next generation comes along. It's already gotten much, much bigger than most of us thought possible even in the heady days of the early 80s.