The failure mode in most cities is availability. In Boston it is hesitation. An emergency consultation here carries a serious front-door fee before anyone has done anything, and the honest consequence is that owners talk themselves into waiting until morning. With an acutely painful, pressure-spiking eye, morning is often too late to save vision in it.
So the useful preparation is not financial cleverness, it is recognition. A suddenly cloudy or bluish cornea, a visibly red eye, a pupil that has gone wide and stopped responding, squinting, and a dog that is simply off — that combination is not a scratched eye or a bit of conjunctivitis, and it does not wait. Knowing that in advance is what converts Boston's expensive specialty density from a theoretical advantage into an actual one.
Worth saying plainly: this is not a common outcome, and most Welsh Springer Spaniels will never have it. But it is the scenario in which living in an expensive city with genuine after-hours ophthalmology is worth every dollar of the premium you pay the rest of the time.