My understanding of the way that the median merge mode works is that it looks at a pixel, and looks at every color value that appears in that pixel across each photo, and then picks the one right in the center of the distribution. This aims to improve the signal-to-noise ratio (such as when there are small grain-like deviations in color values).

If it's being used to remove tourists, then in order for a pixel to come out as the intended subject, the center of the distribution needs to be the color of the subject. So for example, if you took 7 photos, a successful pixel might be:

white shirt, red shirt, sky, sky, sky, sky, black shirt; or

sky, sky, sky, sky, sky, black shirt, black shirt.

If I'm correctly understanding the statistics being applied, then if any particular pixel of the subject is obscured by a tourist >50% of the time (either because there are too many tourists, or because some of the tourists stood still for too long) then no amount of photos can be guaranteed to work automatically, because the centermost value of the distribution can't be guaranteed to be the subject, e.g:

sky, sky, sky, green shirt, red shirt, purple shirt, black shirt.

There would presumably have to be a method out there (perhaps as part of a plugin or third-party software) that would make a lot more sense, and work under more crowded conditions than median does for removing tourists. Something akin to "mode", that picks the value (or range of values) that shows up most often, e.g:

sky, sky, sky, green shirt, red shirt, purple shirt, black shirt.