In my opinion, they lost more from a purely musical standpoint, when Steve Hackett left the band, than when Gabriel left.
Sure, their very best material was recorded with Gabriel, but Trick of the Tail and Wind and Wuthering are only small drop off in quality when compared to the Gabriel era.
But EVERYTHING after those recordings, is IMO, a huge drop in quality. Duke is the best post Hackett album, but even that is pretty mediocre.
In fact, I’d easily rate Hackett’s first half dozen, or so, solo albums better than anything the band did after he left.