Take yourself out to the carpark and slap yourself around.
You were brought in to provide a solution to a need: because you were "useful".
You solved that problem by creating a self sufficient continual solution to that need, which no longer needs you: good news, you are now "super useful".
You have met the need and extracted yourself from serving it. Plenty of highly successful consulting companies are built on that same business model of (1) introduce a high quality problem solver to scope whats needed to solve a probem (2) create a team (as you have done), and (3) extract the high quality problem solver so they are available for winning / fulfilling the next assignment.
I would advise you to report to your bosses this is exactly what you have done. I don't think they will be keen to let you go. You've delivered, and now you're available for the other problems they need solved.
If they don't see the value, bye bye. I would advise you hit the recruitment circuit or form your own business. Now.
I believe if you're always providing value, there will be people willing to pay for it. Too many highly skilled but not so self confident people end up feeling they need to justify their value by creating a reason to stay in a role. That is a resource-of-1 limited vision from the start, and companies aren't stupid: any person that's a critical dependancy is a RISKY ASSET.
Well done on your accomplishment. I suspect you have a very lucrative and continually fun career ahead.