Meh that may be true in some ways, but not really in this case. RTGs were made using a surplus from production for nuclear bombs. As that production ramped down combined with better solar and batteries, the demand went down and it became more expensive to produce them. So not making them made sense.

An RTG is really only useful for missions that go far away from the Sun, making solar non-viable. RTGs are a pain in the neck all throughout the process, are heavy and expensive (even back in the day). The amount of electrical power an RTG delivers is also very low. This is because an RTG only gets warm, nothing more. So we put TEG (Seebeck) devices on the sides to generate electrical energy from the thermal gradient. But TEGs suck ass, they are super inefficient. For example the RTG the big Mars rovers use put out 2000W of thermal energy, but they manage to get only 110W of electrical energy out of that. So if you are near enough to the Sun, solar is the much better option.