Several attorneys have left the firm for various reasons and the firm seems concerned that I may leave also. That's a great question, although a tough one to answer. Although attrition is normal and expected within any law firm, it has a much greater impact when you are associated with a small firm.
Several attorneys leaving a large firm generally has no impact at all.
However, when one or two attorneys leave a firm of twelve, their absence can be glaring. Obviously you are highly regarded or the firm would not be asking what it would take to keep you. But you need to determine if this is the right environment for you and if this is a place where you would like to be admitted to the partnership. It isn't easy to come up with an answer when asked what it would take to keep you. And you are right in thinking that you might be locking yourself in to something if you come up with a figure that is lower than what the firm was willing to pay.