It's not difficult to win at all, if I'm more productive at home, I'm more productive at home, and a smart employer would enable me to choose that. If I'm not, I'd like to have an external space, perhaps the office, to go to and be productive. A stupid employer would ignore their employees and just decide that the office is a universal good.

Now, if you're saying that it's a difficult argument to win with an existing employer who's mandated RTO (rather than a difficult argument to win in general), I'd agree, but I'd say that's true for nearly any argument at any sufficiently traditional, large, or bureaucratic company, about anything. The same place where it'd be difficult to argue for WFM is the same place where it'd be difficult to argue for better pay, dimmer lights, a change in ambient room temperature, less meetings, different duties, less overtime, the use of a mac vs windows pc, a different chair, or any other kind of benefit package, because these decisions get made and then applied without consulting anyone lower in the org chart until those people leave the company and come back asking for them as terms. That's the nature of those hierarchical structures, it's what allows mass layoffs it's what takes agency away from people, nearly by definition.