Working in said situations, the unfortunate truth is that after a certain company size, what I have seen is that the management generally doesn't give a shit about the support load if the customer is in the top 10% of customers generating income.

A hidden cost (besides the obvious "they are paying for the entire support department" cost) is that if the industry you are working with is a niche industry, those customers can also poison the well you are drawing from, even if you expend everything to help them, some are beyond help. Fire them as fast as you can!