I don't think the core concept of reusing designs is the real problem. One of the big problems with reusing designs in the live action films is the lack of color in the robot modes. Back in ye old days the robots were far more colorful so you could just slap new colors on the Starscream design and people would instantly know oh this is another character. You could just look at the colors and go oh this isn't Starscream this is Thundercracker. With such a limited pallet of colors especially on the Decepticon side it's often hard to know am I looking at a mostly metallic colored old character returning or is this some new mostly metallic colored character? With the designs being overly busy you can't just change up something like a helmet for most people to know oh hey this is a different character. Sure the sharped eyed super fans can spot the difference but the casual person who just shows up for films every few years will not notice the little differences when there are just so many parts to each robot. Head swaps can work when you are selling something as simple as a toy, but they don't work as well when you are selling the latest in visual effects. Ugh and whoever came up with generic standard issue troopers for Transformers needs to be beaten with a stick. Outside of Sharkticons it's never been an idea that really works well. It works for Star Wars, it works for GI Joe, it sometimes works for the goons that certain superheroes beat up, but for some reason Decepticons just work better when they are something like the Brotherhood of Evil Mutants rather than HYDRA.