Business continuity (succession planning) - what happens if I die?

I was asked about this by a prospective client earlier today. This isn't the first client that's asked this and I'm growing increasingly concerned because I don't have a plan.

I started my SaaS five years ago and never thought it would grow to the size it has. I have hundreds of clients and more and more very large companies using my application. They, naturally, want to assurances that the whole thing won't fall apart if I get hit by a bus!

The business is bootstrapped and I remain a solo developer with no employees and want to keep it that way if possible.

I know I could try and find a technichal co-founder but I have no idea where or how I would find someone I could trust to take over should I meet my demise.

Is this a common concern? What do you guys do about this? What's the answer?