You have a skill that they need.
It's the old joke about the consultant walking into a room, looking around for 15 minutes then taking a piece of chalk and marking an X on a piece of equipment then charging 50,000 for his services.
Management went ballistic and demanded an itemized bill.
15 minutes work at 400/hr = 100.00
1 piece of chalk = 0.05
Knowledge of where to plac "x" = 49,899.05
Your knowledge of the systems is worth the amount of money they are losing every minute it is down plus the cost to replace it.
Asking for that in compensation is NOT blackmail, it is business...
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