I think it’s alright if the player is at least forced to stand in the open on the platform. You can at least counter him there.
The issue I’m seeing is that Marquis is able to ‘head-glitch’ by standing in a certain spot on the stairs, which covers him almost entirely (expect for his head IE: ‘head-glitch’) ergo making him very hard to counter, especially when the enemy team is decent enough to keep you from pushing in and getting him out of that spot.
There’s also the issue that if he’s getting consistent damage on the sentry it can throw the team off and they rush back to it trying to figure out where it’s being shot from (when they don’t know about this exploit) thus giving the enemy team a chance to push.
If a player is going to solo kill the sentry then I feel that he should have to take a risk doing it. Sitting in a spot that covers everything but your head (which can only be seen from the other sniper perch, unless you push into the enemies side) just doesn’t seem like a balanced strategy imo.
Maybe they could devise some way to force him to HAVE to stand on the platform, or at least further up the stairs to attack the sentry. That way the enemy team can at least see him for the most part, not just his tiny little head poking out from just behind the ledge, (which again can only be seen by standing on the other sniper perch, from what I’ve been able to tell.)