The power loss is due to the fact that the power source is plutonium. Which decays over time. So, your lump of plutonium contains less and less plutonium over t

The power loss is due to the fact that the power source is plutonium. Which decays over time. So, your lump of plutonium contains less and less plutonium over time, and thus generates less power. A more modern generator doesn't really change this basic fact; the power source would still likely be plutonium, and would have the same issue. But yes, you could send up a larger lump of plutonium so that your power budget started out larger and would remain adequate for a longer time.