Basically since the ZEC company decided not to concentrate on ASIC resistance, these ASICs are pure winners since they cost little to what they earn per day and should eventually ROI.
With GPUs its another story. Some GPUs like the 1080Ti, were pulling in like $5/day back in the day with ZEC, but now not so much.
Same with the older AMD Radeon 7970s and 280X, they were pulling in like $2/day and now they are more profitable mining XMR instead.
Hmm, somewhat of a false equivalency; over the time period you are referring to, check the values of altcoins to fiat (not BTC). Earnings have dropped because of Z9s but also significantly because, well, their actual values have as much as halved.