Personally, I'm fine with it. I really like accurate alt modes and in my opinion, the way to achieve that is a fake grill. Plus Hasbro has just been getting better and better at engineering it.
The Earthrise way of folding it all up into the chest was great, and overall the figure does everything transformation wise that I think a G1 Optimus should (except maybe make the front bumper and wheels fold up more somehow, maybe being connected to the back instead of the ass.
Fake parts are antithetical to the idea of Transformers. Make it work with the real parts or don't bother doing it.
I personally don't care because even G1 Powermaster Optimus had an entire fake set of windows and a grill on the back of his truck mode.
"Hate" is a strong term, but I thought it was pretty weird on Earthrise OP, since the fake grill and real grill look nearly identical.
They are significantly different. The bot mode grill has extra detail splitting it down the center which is not on the truck grill. I dislike any faux parts including extra grills going all the way back to the first MP Prime. I prefer seeing the same parts in both modes, but I don't expect designers to drop this trick any time soon.
To me, it depends a lot on whether it would've been possible for them to use the real one. I feel the same way about the chest windows.
I'd prefer the robot abs to be the truck grill, but it doesn't bother me too much when it doesn't happen. What I'd like to see is the bumper becoming the belt/crotch like on the original toy. That's pretty much the only frontier left on the Optimus Prime front - a toy that truly modernizes the original figure. Where the abs are the actual truck grill, the bumper forms the belt/crotch/hips rather than being hidden. Where the waist stripe is the same as the truck stripe and the forearms are formed from the fenders/incorporate the fenders. And wheels on his hips, rather than being stowed inside the waist or on a fannypack. We get plenty of show-accurate updates, and excellent blends of toy and show like ER Prime, but how about a truly toy-accurate one? While I usually go for a balance of toy and show, I'd love to see modern toy engineering put toward making a fully posable update to the original toy.