The most stable element is iron, in terms of having the optimal binding energy (you gain energy for fusing lighter elements into iron, you gain energy for splitting heavier elements into iron). If proton decay doesn't exist, then the trend of all matter would be to turn into iron. If proton decay does exist, then iron itself would decay to less stable elements because the protons in iron would disappear, and the neutron-rich lighter elements would be subject to radioactive decay.

It is unknown how stable our stable nuclei are. Bismuth-209 was long listed as a stable nuclide, until a decay event was observed in 2003, establishing its half life of about 10¹⁹ years (by contrast, the universe is about 10¹⁰ years old).

Memory is a bit rusty on this (30 years since I did nuclear physics) but elements have stable and unstable isotopes (with the same number of protons and a different number of neutrons). E.g. Carbon 12 and 13 are stable and Carbon 14 is unstable (half-life 5730 years). An atom will keep decaying until it reaches a stable form. Then it will stay like that until some nuclear fusion or fission event occurs.

Consequently most chains of decay will reach a stable form long before they get to Hydrogen.

For heavy elements this usually means a terminal at the 206, 207, or 208 isotopes of lead or thallium-205; radioactive isotopes smaller than that usually quickly hit a "nearby" stable isotope.

They're decaying to a stable middle region. In theory, they're decaying into nothingness, but that's like saying "in theory, things reach absolute zero".

Over a long enough time span, even protons will decay.

I don't believe that has been observed. So no-one knows for sure.