You would be correct. There are mobile, semi-mobile and non-mobile elements. Some, like nitrogen are mobile so if there is not enough to go around the plant will take from the lowers and give it to the upper growth nodes. So, if you don't solve the problem by giving more N in this case, the plant will just start taking it form the next leaf in line.I don’t want to take off the shabby leaves because I think it spreads the problem to higher leaves if those aren’t taking the hit. I might be wrong who knows.