For someone unfamiliar with NJPW other than the couple main guys. ..
What is the WWE equivalent of talent exodus?
I'd say it would be something like the WWE having a main event picture consisting of Cena, Reigns, Orton and Reigns. And then Orton and Rollins leaving and Cena has start started to go part time.
NJPW had four top main event guys. Tanahahi, Okada, Nakamura, and AJ Styles.
Tanahashi is the 'old guard' that carried the company in bad times and is now on the way down (kinda Cena).
Okada is the 'next one' who just won the 'passing the torch' match at WK10 (kinda Reigns, but better received).
Nakamura is 3rd on the list, but is still a top tier main event guy (kinda Orton).
AJ Styles was the top foreign guy and leader of one of the biggest faction in the Bullet Club. (kinda Rollins)
The tag team was the top foreign tag team, also Bullet Club.
Losing the tag team isn't a big deal. They can be replaced by others easily enough (see Bad Luck Fale and Tama Tongo already trying to get that spot). It sucks, but they can be replaced. Would be like WWE losing a team like Cesaro/Kidd, but tag teams are easy to replace. The Briscoe Brothers could replace them easily enough.
But AJ and Nakamura basically removed half the main event stars, and a 3rd is on the way down. There isn't anyone who can instantly move up and take those spots, so they have to build some new stars really quick.