I mean, I love the design of the Omega Metroid, it's very beastly especially in its Return incarnation. However, even ignoring the lore it wouldn't have fit the story. The point of the baby being there wasn't to be this huge brute of a beast, it was supposed to be the mutated baby you saved. Having it keep its original form but bigger fits that story beat far more then the Alien T-Rex, and makes the sacrifice it has at the end hit harder since it's a more familiar form to players of the game.
Regardless, you're welcome to prefer a different direction, but understand that just because you disagree with the current one doesn't mean it's not valid. The writers had good reasons to go with the direction they did between the lore, story, and thematic they had and many fans appreciate that direction considering how iconic it is to the franchise. Choosing to ignore all of those reasons just because it doesn't fit your preferences is just putting yourself into a bubble and refusing to let anything in. The reasons are there, and while you may not agree with them they exist and should be given weight.