When I switched employers in 2017, I made it a point to attempt to train my replacement. But that transition didn't happen well. I offered free help for the the first month after I left (new job was in building opposite to old job). No questions were asked. The following month, started getting questions. Offered two options:
1. Pay me directly per hour as a 1099
2. Hire my new employer for professional services, who'd send me in.
They wouldn't do #1 because they couldn't pass that cost to their D0D customer. They mulled #2, then gave up because "fees are too high". It took them over a year to fill my old seat. And my replacement disappears all the time due to Reserves duty.
Bumped into my old manager a year after I left, he said I was doing the job of 2 engineers. Came home, looked at what I was doing at my present employer, realized I was going above and beyond my role, parlayed that into a $25k instant-ish raise.
Then COVID struck. Many don't want to go in to the customer site. Those of us who do have become invaluable. Then add those who've left to switch to FinTech to work from home full time. So I'm doing the work of 1.5-2 humans. Have asked for a substantial raise, have been told it will happen because I carried extra burden last year, same for my team. (Employer is very generous, pays more than competitors, and doesn't have layers of money-sucking overhead management. Company owned by, run by, full of engineers and IT people.)