exactly! if they are happy in there, and nothing else warrants a change sooner, i'd leave them be. it's a lot less worry.
once they start to want to climb over the edge, you worry about them falling out, so you put them in a 'big bed.' then you worry about them falling out of that, so you put up rails. but, there's a gap, and they can get out whenever they want, so you worry about what they will walk into during the night.
then when you think they can handle it, you remove the rails, and worry about them falling out again, and they do...