Short answer: The "worse" final electron acceptor will just not accept the electron, and electrons will just get back up in the electron transport chain, shutting down oxidative phosphorylation.
Long answer: This is related to a deceptively dissimilar question: "Why do cells throw away perfectly good organics in fermentation?"
To really explain this, you'll have to know some quantum mechanics.
Every nucleus, atom, molecule and ion has orbitals (MO theory) , little "slots" for electrons to exist in (shown as horizontal lines). Orbitals have distinct "energy levels", thanks to the electromagnetic interaction between the electron, the nucleus (nuclei) and other electrons. Typically, electrons will exist in the lowest energy level not occupied by another pair of electrons. Electrons release energy when they move down and absorb energy when they move up. A free electron ($n= \infty $) will release more energy moving to the lowest energy level ($n=1$) when that lowest energy level is more negative, and will have to absorb more energy to become free again. This is what is meant when we say an electron acceptor has "higher affinity for electrons".
The ENTIRE gimmick of the electron transport chain is that the electron moves to progressively higher affinity acceptors, releasing energy in the process. By the time the electron reaches the penultimate of the electron transport chain, its energy level is so low/the acceptor affinity is so high (equivalent) that it takes another acceptor with even higher affinity to attract that electron.
But what if the cell just... throw the electron away? Well, that would completely defeat the point. For the electron to become free it needs to be provided with energy. More energy than what the electron transport chain has extracted for sure, because the electron in the food molecule isn't even free to begin with.
Ok, but WHAT IF the cell use the electron to build organics instead? Every cell requires an energy source, a carbon source and an electron source. Unfortunately, that won't work in this case. Organics in biology usually requires electrons with energy equivalent to that of NADH. Since that is the very beginning of the electron transport chain here, it won't work.
The ONLY productive way to throw an electron away is to give it an acceptor and then throw that away too.
So that's why Fe3+ etc cannot accept an electron from the penultimate of the aerobic electron transport chain. Because you'd need to provide energy to the electron to do so.
Back to the question I proposed in the beginning. "Why do cells throw away perfectly good organics in fermentation?". By definition, fermentation means using an organic terminal electron acceptor. What usually happens is that cells do this clever maneuver where they rip the electron from an high energy position in the molecule and puts it in another position with lower energy.
Notice the cycling of the electron carrier NADH.