So here's my summary take:
All pre-season, people were trying to figure out SF's backfield. The team CAN run the ball, but guessing who would do it is like guessing what Belichick would do a few years ago with his backfield any given week. People at least consensus agreed that Mostert was fragile but has put up really good numbers when healthy. Hasty and Mitchell are just guys. Sermon has talent, no experience, and was a 3rd round draft pick - not an insignificant pick here folks.
Week 1 Sermon is a healthy scratch - this is far from unheard of for rookie RB's. Blue chip RB's often don't play on special teams in college for fear of getting hurt, so they don't have the technical skills to play in the NFL. They often haven't developed their blocking skills to the NFL level yet either. I forget who it was, but a year or two ago, there was this exact situation on another team. Guy was a rookie, and was a scratch because he didn't play special teams and couldn't block well. Also - in week 1, Mostert gets hurt. The oracle was right. He's fragile.
Week 2 Sermon is active. With Mostert out, his role gets elevated. In my opinion, were it not for his injury, he'd be well on his way to a lead back role on running downs. The other guys still get action - maybe they're better blockers (don't honestly know), and they still play a special teams role...but the team now needs a guy to run the rock. You don't draft Sermon in the 3rd to sit him. Sermon takes a big nasty ILLEGAL hit and is out. How do you fault the guy for that??
So now here we are in Week 3. What's permanent? Mostert is out for the year. Neither Hasty or Mitchell put up massive numbers against a middle-of-the-pack Philly defense.
So honestly - can anyone explain why Sermon isn't high on an RB radar that in any good league is devoid of talent outside of the top 10 or so guys? He's rapidly on his way back from a concussion due to a hit that wasn't his fault. The starting RB (the guy that kept Sermon from going high in drafts to begin with) is hurt and done for the year. The other 2 RB's are just guys. Sermon is going to get his shot. If he lives up to his pedigree and draft pick, he'll win this job outright. At WORST, that would put him around RB 20-25, which is where Mostert went...but I'd argue much higher because Mostert was tempered by knowing that Sermon was around and Mostert is fragile. Now, it's just Sermon. No Mostert - to me, that would've warranted at least a mid-teens RB grade for fantasy due to opportunity alone. Would we even be having the conversation re. whether to add Josh Jacobs or David Montgomery if they were available in your league?
If he's available, Sermon is probably the best chance at a "league winner" at RB out there right now, maybe barring Chubba depending on McCaffrey's status - but at least we KNOW Mostert is done for the year.