You need to listen to the 11 Personnel podcast. Jourdan goes into depth why she (and to be honest, so did I) think it was a high probability they'd keep Perkins.
But back to acquiring Freeman. So does he take Hendo's place as RB1? Michel's place as RB2? Almost certainly Funk will be on STs and return KOs. Is Freeman going to return kicks? Or do you think the Rams suit up all 4 RBs on game day? If you're not suiting up all 4, then you're willing to let a guy like Perkins go for a guy who may not ever even be a game day active?
This is the problem with this line of thinking. The numbers don't work.
As for Perkins. The Rams have put a lot of training into Perkins and he is thriving. He just gets better and better. He has gained huge experience this preseason, and did a good job. Perkins wasn't retained because the Rams want 2 back-ups. He was retained because there is no more valuable position than QB. And when one with his potential shows up, and begins to prove what he can do, you don't lose him.
So do you let a guy like Perkins go in order to keep a 10th OL? A 10th LB? A 7th WR? Or a 4th RB? Heck no. That's a really poor value equation. Jourdan says she knows for a fact (not her speculation) that there were teams lined up ready to pluck Perkins if the Rams released him for him to become their back-up QB. That's right... they're QB2. That's how much he has impressed other teams.
You find a way to keep your best talent. Period. And when that talent is QB talent? You don't ever take chances with it.