Just to clarify, this is incorrect. We never had 250,000-500,000 players. The highest peak was also not 2015, but actually just last week.
The highest active live player count until last week was last year in 2020 with 121,784: https://warthunder.com/en/news/6675-special-award-for-record-breakers-en
Last week, we hit a new all time record of just over 130,000.
So no, the game is in fact not dying, but actually making new records