teeth are designed to erupt as they wear down. when you are missing one
of the teeth the opposing erupts.
Sometimes depending on how your teeth are setup a portion of that tooth
is held by the opposing tooth but the rest of the time the tooth will
continue to erupt until it hits the gums. This can interfere with
function when you move your lower jaw side to side it will collide on
this tooth. If you do not restore the lower in a timely manner you will
not have room for the crown and may need to have root canal done on the
upper with a buildup and crown. and if you wait even longer you may
lose the upper opposing tooth as well.
My suggestion is if you have enough bone on the lower jaw that is above
the nerve get an implant.