My former coworker called it “making your own ball and chain”, and he is a prime example of this as well. He was very good as SAP and ABAP development, so most SAP work would get assigned to him because he can have it done faster than anyone else. Which also means no one else is getting much experience or training on SAP. And because he is always busy with SAP, he never gets trained on anything else either. Leaving was not a realistic option due to lack of experience in any development outside of SAP and SAP setup and customizations being very specific to the industry and the company.