I'm not necessarily advocating for Bell to get that kind of extension. It's something that I feel very conflicted about because there are legitimate reasons to extend and to let him walk. Right now, I'm in the franchise tag camp more than any other. We have a ~3 year window and we can keep Bell for 2 of those years via the tag (even though it's probably not a good idea from a fiscal standpoint to tag him a third time).
Having said that, I completely disagree that Devonta Freeman can do what Bell can. While the jury is out with regard to the long term impact of Bell's usage, at least he has the ability to handle that kind of workload for now. Freeman is a complimentary back whose performance isn't even appreciably better than Coleman's. Freeman has a study frame and is more capable of running inside than most smaller backs, but it's hardly a strength. He's useful in the passing game, but nowhere near the asset that Bell is.
As for the culture issue, while that may be a valid point, it's rendered moot because players can only influence the culture whereas the coach is ultimately responsible. If Tomlin has fostered that kind of culture, removing a player or two isn't going to change anything.