The decks that play Sage usually have a way to search it/dig for it specifically so that they can kill enchantments—RiP for CoCo, Moon for Valakut, both for Knightfall, etc. Sage is also a common board-in for these decks when they expect Worship, and killing an Aether Hub can be awesome against Affinity in some games. It’s just a way more flexible card with far-reaching applications, which is crucial in Modern. Manglehorn will never replace it.

You could, especially if there are more coming, but I don’t know why you would. It would mean dramatically retooling the mana base and with that shifting the creatures you run. If it is worth it I suspect that rather than completely rebuild existing lists a new, probably Esper, list will emerge.

I completely forgot about the enchantments part, it just comes up so rarely. Which still makes me think that in most cases Manglehorn would still be better.

The way I look at Gideon is as a 3-mana modal spell that’s a 4/4 indestructible or a Pacifism, with an “Oops, I win” button tacked on for certain matchups.

The only thing I think you didn’t mention is that a couple of the cyclers look pretty promising for Living End. Now that there are some powerful one-mana cycling options in white and blue, maybe it’s time to replace Demonic Dread with Ardent Plea?