Last time they said anything official the number of players was actually rising.
I know a ton and the grand finals was sold out each day and viewed by almost 400 thousand players in twitch only, not counting Disney channel, and all other channels that had it running. Plus the league was a huge success so much that 4 new teams are already confirmed plus the original ones, its only growing and a lot.
Las time i checked the active numbers were in the hundreds of thousands per day and per region, there are literally millions of players playing the game around the world every day.
New maps, new heroes, new social systems, the entire Overwatch League, tons of balance changes all the time, plus all the new things they are working on right now.
False, the game and the league are growing and are better than ever, what you say is a myth spread by haters nothing more.