Reddit mobile site does this when you tap a post. It drives me nuts. Then you have to hit back, which refreshes Reddit and brings you back to the top. I think it’s intentionally designed to get people to switch to the app.
I doubt they'll care. By that time, they'll be confident that whatever loss they incur by killing old. will be worth it for them. If there was an actually significant user base using old., I imagine "regular" Reddit would look a bit different than it does.
And that is just the visible part, actual clickable area is 1px by 1px so even when you correctly click on the "x", you don't actually close it.