Technically, there is work required from Frontier, because they have to now maintain two versions of the game and BGS. However, I would suggest there is no extra work required from Frontier, because they've already pledged to do this for console.No work required from frontier.
The one argument I've seen that has merit is the idea that Odyssey can never fully become all it's meant to be as long as Horizons remains legacy. If a person believes that maintaining a legacy version of Horizons is holding back their beloved Odyssey, I understand why they would push for unification of the code base. But again, the console version of Horizons is the Kryptonite to this argument, as long as consoles share the same BGS as PC.No harm to you. Doesn't impact you.
The good news is that the naysayers have no more influence over Frontier than you or I, so whatever Frontier decides to do, they will do. A determining factor will be the number of players regularly logging into Horizons vs Odyssey, especially those who own Odyssey but continue to log into Horizons instead. Would Frontier take the axe to legacy Horizons if it has the majority (or even a large minority) of the player base?