It would be wonderful to have lifetime job security, wouldn't it? But that's not possible with a not-for-profit organization. Why? Because not-for-profits are governed by a board of directors who alone have the right to ratify hiring decisions--particularly at your senior.

As stewards for the organization, they have an obligation to ensure that they are making decisions that are in the best interests of the organization.

Should it come to pass that you no longer have the skills that the organization needs, the board needs to have the power to replace you. To the extent that anything can provide lifetime employment, you can do that in two ways: Form a for-profit company and be its sole owner, or maintain an excellent rapport with your board and provide such superlative service that they won't want you to leave.