I made an offer to a long-time client to build a system for them that would increase their profits drastically. They agree at that point. The problem is if the system completely broke for a few days, they would lose a lot of clients. They have confidence I can develop/maintain it to prevent this. I feel like our relationship is such that, they know I wouldn't just disappear like some consultants.
The problem is, they are worried about if something were to happen to me, what then? It would be a matter of time before an update broke something they couldn't fix. I would provide all the source code and it would not be hard for another developer to replace me (NodeJS codebase).
They couldn't learn it in a few days enough to be useful most likely, but the odds of me dying and the system failing at the same time are so low. I'm not old or otherwise at extra risk of death. I feel like this is such an odd concern they are using it as an excuse? Whats a professional way to approach this? I really want to build the system because it actually deeply interests me.