It’s hazardous to think of customers in the plural, as if they’re really a homogenous group that exists in the aggregate and can be thought of and served as a whole.
The better way to think of customers is as follows: There's just one customer, the one you're facing right now.
There's only Jim. One Margo. One Alecia.
Let your competitors keep thinking of customers as an abstraction. You need to think of them, and serve them, in the specificity of their Jim-ish, Margo-ish, and Alecia-ishness.
Jim, who likes his service languid with plenty of time to consider his options. Margo who is always in a hurry, and doesn't care how your day was. And poor Alecia, whose cat is at the vet, and isn't in the mood for your Pollyanna ponderings.
Now, every customer's different from the next one -- Jim from Margo, Margo from Alecia, and Alecia from Jim. Some will be easier to serve, and some harder. And some are easier to serve sometimes and less so at others. Regardless, I suggest in the strongest terms that you think of every one of your customers as a core customer—and treat the loss of a customer as a tragedy.
Here's why: Because every single customer is irreplaceable.
Regardless of the size of your market segment, once you start writing off customers, I can predict the day in the future (and it's probably not far into the future) when you’ll be out of business.
If Margo leaves, she's gone, forever. That Margoish opportunity has evaporated for the length of your enterprise. Your available market has diminished by one: one you already had on your side.
And this is a calamity to be avoided.By the way, the same thing holds true in B2B companies with large accounts. Stop thinking of a customer called “IBM.” There's no "IBM account." There's only Juli in A/P, Jeff in design, and so forth. That’s the customer you need to stay close to, not an abstraction called IBM.