Starfleet could easily flat out refuse to let Kirk the Savior to ever set foot out of a padded and shielded display case, uh, office, again. John Glenn was refused further spaceflights, and so was Yuri Gagarin...
But neither Glenn (who did return to space, in 1998) nor Gagarin were Defenders of All Life on Earth - a function Kirk could not perform other than on a spaceship.
As soon as the decision was made to finally visit present-day Earth in TMP
, the default plot for further movies became Threat to Earth
; not only TMP
but the fourth, eighth, and tenth movies (as well as the Abrams movie) had this plot. Naturally, this implies Kirk (or Picard) Must Stop Threat to Earth