It actually makes sense I think. Fly half is a position where the "peak" age of a player has really come down quite drastically since professionalism kicked in. Mid twenties fly halves are now often the ones outperforming older, "more experienced" heads. It's rarer to see a 30+ year old fly half starting regularly at 10 on the world stage nowadays. Just look at the 6N. Starting fly halves were:
England: Farrell (27)
France: Ntamack (19)
Ireland: Sexton (33)
Italy: Allan (25)
Scotland: Russell (26)
Wales: Anscombe (27)
I put this down mostly to the fact that there's a special quality/attribute to a fly half that just comes naturally (and is maybe also developed from a young age with early exposure to a rugby ball in the back garden). There's something innate about a quality fly half in their "rugby brain" that means age becomes less of a factor - and so why wouldn't you pick a younger, quicker, stronger player in that position who has the same composure as an older player, who would be traditionally thought of as being a more experienced player with a calmer head.
Marcus has those qualities - he's old beyond his years and has real composure. Sure, he has a lot of growth still in him, but I think the days of "30+ = experienced" are gone. Marcus is already on course to break Brownie's apps record for us by a long distance - assuming he too is a one-club man.
What's more important to me is we have different styles of fly half so we can change up our game from the bench. I don't know loads about Herron but I think (don't quote me on this) he's quite physical - which would give us something different to Smith/Lang.