If the masses (esp meaning those who are not firmly aligned to a particular club) buy the merch of other clubs because they are consistently seen to be more successful, then it must have an impact on the commercial side. I imagine the new football fan coming in will not be choosing United at the moment.
I get what you're saying but I have always found it odd that sooooo many of my friends and workmates support Liverpool, by far more than any other club and yet they haven't won the league in 30 years so you would have to be around 40 years old to actually know what it is like to be Champions and most of these fans I see wearing Liverpool gear aren't. It doesn't appear to have affected people picking them as their team. I suppose in that 30 years, they have consistently still won knock out competitions.
United are still seen as one of the biggest clubs in the world and I remember during their drought period up until 1993 that they were still a massively supported club so maybe the current form won't have that much of an effect.
Not as much exposure in those days compared to todays social media-- Facebook Twitter etc.