will everything in the universe be lead or carbon, eventually?
I don’t believe so.
While it appears to be possible, when you are talking about all the matter in the universe, you have to bring into the equation many factors that would prevent that from happening.
On the simplist level, the “universe” as we “know” it would be altered to a point that decaying wouldn’t take place as we “know” it, so that decaying’s final run at decaying everything into carbon would never be realized.
Actually, iron is the most stable element. Lighter elements release energy by fusing to form heavier elements, and elements heavier than iron release energy when they split. You really only have fusion occurring inside stars. As the universe expands and becomes less dense, all atoms are more likely to decay than to run into another atom to combine. If protons themselves decay, given enough time, the universe will be left with nothing but photons, and eventually they will become so sparse that it will be dark and cold with nothing ever changing.
Try not to worry about it too much, it won’t happen for a long time.