So the ecologically soundest choice isn't so much between heater and pump, it's between 'birdbath' and a better designed water reservoir that won't freeze so easily in the first place. Obviously people who live in really tiny yards or who only have balconies don't have much choice, but even on my dinky 5000 sqft lot, it's perfectly reasonable to put in a pond that's about 5 ft diameter and deep enough to be below the frostline. People who don't want to be water suppliers can supply something else. It's the net resources in the entire neighborhood that counts most. Pumps aren't really that much of a hassle if you get the right one to start with.
Modern pumps are sealed, so there's no oiling, just an occasional filter clean out.
They can easily last for years. Like many things, it's responsible enough in moderation. If the proverbial 'everyone' does it rather than looking into the alternatives, though, it might become less responsible. Tiny things can add up into big things when summed over a large population.