On a preplanned plot, the players having "more control" isn't necessarily ideal. In a basic game structure common to many tables, the DM prepares a plot-based game and there's nothing else much outside of it to engage with (except perhaps killing some time with intraparty banter and of course shopping). So incentivizing the players with XP to go off the script isn't a great idea. Instead, you use story-based advancement with clear "significant goals" so the players engage with the prepared content only.
Yeah, but that is forcing the players mechanical and story wise to stay on course. As a player, I wouldn't like that, to be forced to not stray away a little, not to have the opportunity to do anything else in the world. Such a game would feel like a dead world, where there is nothing outside the walls of the plot.
For playing D&D you need to create an Illusion, an Illusion of a living, existing world where you have agency in. With milestones and a fixed plot, you are killing this illusion. Your choices don't matter. You follow a preplanned path of encounters and if your dice a lucky, you succeed.
With having XP you are saying that there is the possibility to go off the rails, even if the players never take that opportunity, you say mechanically, the world is open to you. The illusion of an open world is stronger, the illusion that your choices matter is stronger.
With Milestones on the mechanical side, you take away that illusion. Your choices don't matter, the world is not open for you to explore.
The more railroady the story, the better XP help with the Illusion of choice. It will increase the game experience for the players and create a stronger illusion of a living world.
I would quibble with calling a plot-based game a "railroad" though if the players have consented to following the plot to the best of their understanding. Which I think is a good topic for Session Zero if the DM wants to run such a game. Plot-based games are generally a lot less prep than a sandbox, so it's common in my experience for DMs to gravitate toward this structure. It's not my cup of tea because I find them very strange to run, but then I have more time to devote to prep than others.
It all comes down to the Illusion of choice. Players usually hate it, when they feel that they have no choice. Even if they agree to a plot based game.
On the mechanical side, milestones don't give an illusion of choice, XP do.
So even if it is a little easier for the DM to use Milestones, XP will improve the feel of the game, improve the illusion of an open world full of adventures that is for the players to discover.