Imagine having that customer and then them saying they're considering dropping you in a year if you don't have a particular feature that's not on your roadmap (or maybe not even that related to your product in your opinion). Do you drop everything to please them, or do you stick to your plan? How much pain do you endure to keep the one big customer happy?
I've seen close friends work in companies with 2-5 large customers, and the team regularly endured significant pain to keep one of them happy because it would be business-changing to lose one.
Unless you are very intentional about how you handle this sort of thing, it'll be bad by default. Don't take this as discouragement, but do make sure you really know this in advance.