That's a nice platitude and all, but it's not literal. Madokami is a law of physics, not an emotion. The fact that she's a sentient law of physics doesn't change that.
Not to be pedantic, but in the PMMM!verse, Hope and Despair are both emotions and metaphysical laws. They're also energy.
Madoka is the concept of Hope. She is the propagation of space. She is the birth of a new universe. She is Salvation and the reward of all Magical Girls so they can smile in the end. She is deliverance, and she is the denial of the darkest of futures.
She is literally the personification of the idea that things will be better, and it's not over for you. Yes, she only applies to Magical Girls because that's what's relevant, but Magical Girls are just humans given artificial access to humanity's birthright and potential. What happens with the Law of Cycles if, hypothetically, humans figure out how to use magic without Kyubey?
There's honestly infinite possibilities for how things can develop with the universe in the trillions of years it has to reach Heat Death even if Kyubey quits his job right now. If Madoka says things will work out, I'm going to take her word for it. If it involves information the audience and Homura-as-viewpoint-character doesn't have yet, we'll have to wait and see. But I can literally speculate about assloads of possibilities that don't require Madoka to be a liar.
And considering the universe got rewritten twice in a month as of Rebellion, saying Madokami's too limited to deliver on any of these possibilities is irrelevant; she might not do so under the familiar post-episode 12 status quo, but Rebellion's main point was showing off how much this is likely to change if the Law of Cycles is allowed to interact and behave in any kind of abnormal way.
Madoka is a Goddess bound into the rule that is the Law of Cycles. When the Law of Cycles is no longer needed, when the cycle is finally closed and no longer need spin, what becomes of the chains that bind her actions? When she must no longer eternally fight, what does she do with her infinite time-beyond-time?
What does one do with all possible magical powersets, full access to all time and space, and absolute no obligations or responsibilities that restrict what you can and cannot do?