A line captain leaves. The company is up for the expensive of selecting and upgrading an FO to Capt, and also the expense of recruiting and training a new FO. The more qualifications you have, the more expensive to replace. A TRE leaves. The company is up for the expense of selecting and training a TRI to TRE. Selecting and training a line Capt to TRI. Selecting and upgrading an FO to Capt. Recruiting and training a new FO.
The "if you don't like it leave" attitude is expensive.