I think this is a good (but hard) opportunity for building the rest of your team. You're finding a real vulnerability in your team's structure, and being proactive like you are is a good way of fixing it.

First, figure out exactly what makes this person so critical - it may seem obvious, but break it down into specific tasks and skills. Maybe get them to write down the things they do for each of these that took the most time to figure out. Second, evaluate who else on your team can pick up a specific skill or task, or who would best be able to grow into that particular role element (rollette?). Finally, spend some time and sweat encouraging the rest of the team to grow into these areas - make sure to show them you appreciate their taking on the challenge. If no-one can step up for a specific rollette, you may need to start recruiting.

Kudos, by the way, (as anigbrowl pointed out) for wanting to do the morally right thing. Good luck