We all love the certainty of the solid revenue that comes from a big customer. It gives us credibility with other prospects. It pays a lot of salaries. Occasionally we reflect on the risk to our business if they left us...most of the time we don't.
Yet the tyranny of the major customer can help us hide from the real foundations of creating a repeatable sales process or building products that many customers want. It can keep us too focused on the needs of that major customer.
Why not play out the 'what if' scenario, that you fire your major customer, say 12 months from now. Examine the revenue gap - plan what you would do to fill it. What would your new customer acquisition strategy look like? What sales activity would you plan? How would you strengthen your team?
It always amazes me how resilient companies are when big customers move away. There are worrying days and possible layoffs.
It's much better to plan for the breakup before it is imposed on you unexpectedly. Assess your reliability on your major customer. Are they an oppressive power? If so, then fire them. It will make your business stronger.