|Not sure I really understand your question.
The problem doesn't really seem to be you but rather them. They need to find someone to support the product.
Best I can see here is that you are attempting to find that support for them and then get them to agree to it.
For that scope of work you mention there are only two possibilities.
# A single individual
# A smaller consultant company.
The first case does and always will have the single point of failure. There is no way around that.
The second will cost the company more. Again no way around that.
The first case could be mitigated but only if someone in the company became actively engaged in managing the process and if the problems were never immediate. Then they could hire one or two college students. Those students would be looking for real experience and would cost less. With active management then replacing them becomes a management problem and not a technical one. So if one disappears then the management process finds another. It must be active management because they will disappear.