I don´t think it´s too fair to compare the transition from Gears 4 to 5 to the transition from Gears 1 to 2.
Gears 2 was the second installment in the franchise and Gears 1 was good but needed some polish.
Gears 5 on the other hand is the fifth entry in the main-line. I personally don´t want it to change the core gameplay but I think some bigger changes could have been made. However, I believe to many it´s more about the aesthetics. Graphics could be better but yeah, it´s hardware from 2013 and by now it´s become barely possible to make giant leaps in graphics. We will never see such a leap in graphics, as we did with the transition from Gears 1 to 2. These times are over. However, I think people are generally not too happy with the Gears 4 artstyle. I can´t really say what it is, but Gears 4 looks way too clean. It looks outright cartoonish at times, which is due to the characters (Even the normal ones, not just all that colorblast etc.) and some design choices. For example, that DB metal just looks…boring? This blue-yellowish DB stuff. From what I see, many don´t seem to dig that style. And then the first map they show, is this training ground, which is again, more of that DB stuff.
But they made some improvements already. The Armors are thicker again, resembling old-school Gears more and they seem to have got ridden of all the cheesy designs of Gears 4.