Not to trample on the "harem ending" ideas, but take into consideration that this runs in
Weekly Shounen Magazine, so it's highly unlikely that it would promote polygamy to the teens. From all the titles from the magazine I have read, unless I'm forgetting something, never even once I have witnessed a definite harem ending. An open ending is not a harem ending by the way, and that also pretty much out of the question here, unless Fuutarou runs away from the altar.
So instead of thoughts like "This will be a failure, if it doesn't end with a harem, cause I started reading because of that!", try to adjust your expectations to something like "It would be so good, if this ends up with a harem, but I should only expect that possibility with caution, so if it's doesn't happen, it won't let me down that much."
Peace, have a nice day!