I don't expect game developers to make a unique monster for every stage of progression. Especially not unique art. That's a lot to ask.
And therein lies the inherent weakness of video vs table top games. tabletop games don't require massive artwork. so you don't have to compromise on the design because you don't have the artwork manpower to execute it.
By holding video games to a lower standard you encourage mediocrity.
"Its too much artwork" is not a legitimate excuse for lack of requisite variety in enemy types.
Mediocrity? Have you played the games I mentioned and examined how much work went into the enemies in them? Your responses seem separated by a few degrees from how I'm judging these games.
With enough thrust, even pigs can fly. But when the thrust inevitably runs out, those pigs fall to their death.