I think the decline in instrumentals goes hand in hand with the declining interest in dancing. When I was growing up, going to a dance was the thing to do. Not much these days.
OK, here's Top 20 instrumental hits from 1980 to the present(?), in chronological order:
5/31/1980 | #18 (2 wks.) - Meco: "The Empire Strikes Back (Medley)"
8/8/1981 | #10 (2 wks.) - Mike Post: "The Theme from 'Hill Street Blues'"
10/17/1981 | #10 (2 wks.) - Royal Philharmonic Orchestra: "Hooked On Classics"
11/28/1981 | #1 (1 wk.) - Vangelis: "Chariots of Fire (Main Title)"
3/16/1985 | #3 (3 wks.) - Harold Faltermeyer: "Axel F"
8/10/1985 | #15 (1 wk.) - David Foster: "Love Theme from 'St. Elmo's Fire'"
8/24/1985 | #1 (1 wk.) - Jan Hammer: "Miami Vice Theme"
3/21/1987 | #4 (1 wk.) - Kenny G: "Songbird"
10/15/1988 | #13 (1 wk.) - Kenny G: "Silhouette"
10/14/1989 | #11 (1 wk.) - Jive Bunny & The Mastermixers: "Swing the Mood"
5/4/1991 | #11 (2 wks.) -
David A. Stewart & Candy Dulfer: "Lily Was Here"
12/5/1992 | #18 (2 wks.) - Kenny G: "Forever in Love"
4/27/1996 | #7 (2 wks.) - Adam Clayton & Larry Mullen: "Theme from Mission: Impossible"
12/25/1999 | #7 (1 wk.) - Kenny G: "Auld Lang Syne (The Millenium Mix)"
11/23/2002 | #2 - Kenny G: "Auld Lang Syne (Freedom Mix)"