Some sort of philosophy occurred under poganism, and the beginnings of science, but how many sensible people ever seriously believed a word of all that animist bilge? Obviously, when it suited the doddering Roman Empire to compromise with Christianity it was bye-bye Christianity, hello God!
Ehe... what? Pagans fully grasped the concept of metaphor. Don't forget that a Pagan could be an atheist. A few notable Greek philosophers for instance. They didn't stop being Pagan. Pagans had a philosophical rational around all their feasts. If you study all the feasts they had lots of aspects that are great. I forget which one, but the Roman's had a yearly "being honest day". Where you were allowed to be perfectly honest with whoever and they had to accept it and if necessary forgive you. And anything said on that day you just had to let go of and move on from. Imagine how great it would be to have some of that in today's world? There's no magic required to see the wisdom of that.
Or the Baccanalias. One day a year when it was ok to make a complete drunken fool of yourself. Not just ok, but encouraged. This should be put in contrast with the very stoic ideals of the Roman empire. A real man was in control of his emotions. It's not hard to see the upshots of Paganism.
Like I said, what we mostly hear about Paganism is stuff that's filtered through Christian anti-pagan propaganda. And there was a lot of that.
I think you're reading history completely wrongly. Constantine didn't dodder. He did what was politically necessary to keep the empire together. And by the look of it he picked the right moment. And once he shifted there was no hesitation. We know from archeological evidence that Constantine just added Christ as another god and went on with his Pagan ways as usual. But that was pretty common back then.
I think you need to let go of seeing Paganism as a archaic religion or unsophisticated religion. It's on par with Hinduism. Christian theology is dumb as bricks in comparison.