No universal rule requires the employee to remain the only knowledgeable person. Assess the actual knowledge and proposed duties.
Explain the continuing assignment
Training others may be part of the work, but identify what the applicant does before and after the handover rather than relying on an unsupported irreplaceability claim.
The knowledge must be of the petitioning company's own product, process, technique or procedure, or an advanced level of knowledge of its processes, rather than expertise the wider industry shares. Within the relevant preceding three-year period, the employee generally must have completed one continuous year of full-time work physically abroad for a qualifying organization in a managerial, executive, or specialized-knowledge capacity. State the standard plainly and without inflation: L-1B requires specialized knowledge, meaning special knowledge of the employer’s product, service, research, equipment, techniques, management or other interests and its application in international markets, or an advanced level of knowledge of the organization’s processes and procedures. Uniqueness is not required, so there is no need to claim that nobody else understands the system; what is needed is a factual description of knowledge built inside the organisation over time and applied to identifiable pieces of work.