>As for HFR content being "easily" downsampled - it really isn't, at least not with respect to motion blur

Er, why not? Motion blur is simply time integration. Why can't you simply average adjacent frames during your framerate decimation?

In fact, since "real" motion blur from shuttering uses a likely-suboptimal square windowing function (shutter is binary open/closed), I'd hazard it possible that motion blur computed from a higher source framerate could potentially be more perceptually pleasing (by using a smoother windowing function). You have more information to work with, after all.