Dude, that's bullshit and you know it. All of their record sales plummeted.
In the '90's, 80's metal was considered cheesey. Guitar solos were a thing of the past. Core fans don't make for record sales. Please don't pretend this isn't true.
Such absolute, total bullshit.
Take a look:
Slayer's Divine Intervention came out on Sept. 27 1994 and sold 93,000 copies in its first week. It was soon certified gold and was Slayer's biggest success to that point.
Diabolus in Musica (Slayer's weakest album, imo) came out on June 9, 1998 and sold 46,000 copies in its first week. It has since gone on to sell 290,000 copies. Most would agree that this is, by far, Slayer's weakest album. Yet, it still sold reasonably well.
Megadeth's Countdown to Extinction was released on July 14, 1992 and went on to go double platinum, selling over two million copies.
Youthanasia was released on November 1, 1994 and went on to go platinum faster than any Megadeth album ever.
Cryptic Writings was released on June 17, 1997 and was Megadeths's six consecutive album to be certified gold.
Risk was Megadeth's next album and was the closest thing to a failure they have ever had. Most would also agree it was their worst album.
Notice a pattern here? The good or decent albums by both bands sold well, the ones that were not good, did not sell. You can blame Anthrax's lack of sales in the 90s on the "metal scene being dead" all you want but had they made
good albums they would have sold fine.
Anthrax bombed in the 90s and beyond, not because metal was dead but because they changed their style and the new music, with a few exceptions, stunk. It wasn't just because of Bush. Hell, it probably wasn't even mostly because of Bush. It was because they decided to change their style and go with the trends.
By the way, I have never called Bush a grunge singer. To the contrary, I think Bush is a great metal vocalist. I love his stuff in Armored Saint. But his Anthrax stuff is sub-par, mostly because the music is sub-par. Not entirely his fault but there you go.