Why wouldn't he live? Biden is older.

Constitution doesn't state anything about running for office.
22nd Amendment could be repealed.
It could be revealed that any Amendment ratified after a certain year, like 1871 or 1913, is voided due to illegitimate corporate government takeover of the United States.
Trump could just run, win, EC elects him, SC sits on their hands, and nothing happens and the people move on with their lives?
There's more scenarios...

Surely some of the greatest minds didn't think that through... There were some who wanted term limits, but our system allows the people of the United States to vote out who they don't like (see: Carter, Bush I, Biden) Washington was tired of politics and was getting old, and some of his buddies stuck to tradition.

The framers of the Constitution couldn't agree on term limits for the President, and the US had no problem until WWII happened.

If The People like a President and approve of the work he is doing, then they should be able to continue voting him into office, if they don't then they can vote him out. There was no American King for the first ~150 years before the 22nd Amendment.