Quest markers have a place in open world games with NPC schedules as long as a few pre-requisites are carried out. If you're told, "Go talk to Ontus Vanin in the Imperial City" as a quest objective, the quest marker shouldn't just point you straight towards him. I'm quite happy for a marker to appear at the Imperial City without any other context and essentially function as the quest giver having marked the location on your map. Then you go to the Imperial City and either need to search without further direction, or you can talk to NPCs to get further directions. Maybe some don't know him, maybe some know where he lives and can direct you to the district of the city (marker added to Talos Plaza) or you might have been lucky enough to stumble upon a friend of his, and with a high enough disposition they will direct you to his home (marker added to his house). Maybe others know his general schedule, like the merchants of stores he goes to on a regular basis. If you ask the owner of the Mystic Emporium about him, they might be willing to tell you that he shops there and at a few other stores on all days of the week between the hours of 2pm and 8pm (conditional quest markers added to all of Mystic Emporium, Rindir's Staffs, Gilded Carafe, Main Ingredient, First Edition during those hours). Finally, speak to the owner of an inn/hotel in the city and they might confirm what time he eats at their establishment (conditional quest marker added to Tiber Septim Hotel between 12-2pm, and Foaming Flask between 8pm-12am for weekdays only), or if speaking to the owner of an inn he doesn't frequent, they might know which ones he does go to (markers added to Tiber Septim Hotel and/or Foaming Flask with no conditions). Ontus Vanin is also a former Battlemage at the Arcane University, so some citizens may direct you there (quest marker added to Arcane University) and members of the university may be able to direct you to any one of these clues, having been colleagues and knowing a little about his schedule.
Once the player has gained all of this info, or perhaps something like 4/6 or 5/6 clues, a marker could appear directly on Ontus Vanin so that the player can beeline towards him in case they've missed him somehow while gathering clues. Maybe it doesn't need to be that complex, but just some work done by the player in order to be directed towards NPCs especially if they're the kind to move around a lot.
The problem is that this requires a lot of voice acting, and nothing about the NPC's schedule can change after that VA work has been done. That's why you get quest markers. It's not just an accessibility issue.