Couldn’t we have a lead box lined with these radiation to electricity converters with a small amount of radioactive material in the center, and have an energy generating device that would last for thousands or even millions of years? Imagine putting the sun in a box lined with solar cells, but on a much smaller scale.
Is there a reason this wouldn’t work?
So you need to consider the relationship between the amount of decay radiation and how long a substance lasts. The more radiation, the faster the fuel will decay. If you want something to last a long time this way it will probably be too stable to generate a lot of energy.