Did I go to an unnecessary expense? Should I have just left the 20a
breaker? I can't think of a single situation where leaving the 20a breaker
would have been unsafe. Nothing but the garage door opener will ever plug
into the ceiling outlet (with the #14), and it is all exposed to the air, so
it is unlikely to overheat dangerously even if I did. Whatcha think?

I think you knew the answer or you wouldn't have replaced the breaker.
And since nothing will ever be plugged in except the opener, I assume
that means you'll live in the house until you die, preventing anyone
from using the outlet for anything else, and have the house bulldozed
upon your death, correct?

Jeff