You are almost correct. SNL was actually at its funniest shortly after Chase left, but then began a rather rapid descent when several others left for bigger and better things. SNL was at its very best when the "Not Ready for Primetime Players" consisted of:
Jane Curtin
Dan Ackroyd
Bill Murray
Lorraine Newman
John Belushi
Gilda Radner
Garrett Morris
I'll never forget the Weekend Update segments with Curtin and Ackroyd (their Point-Counterpoint was hysterical), the commentaries by Emily LaTella (Radner), and sports coverage by Manny "Baseball been berry-berry good to me" Mosa (Morris). Also, the Bad News Bees, the Samurai sketches, and other classics from Belushi were great.
When that ensemble broke up, SNL went straight into the toilet.