I sometimes feel like a 'failed' mum too - well not really failed, as I know I am a fab mum to my boy, but more that there must be something missing in me for not wanting another. I'm part of a book group and several of the mums have just had/are pregnant with their third baby, and when I'm with them I always feel I'm lacking 'maternally' for only having (and wanting) my boy.
I occasionally get asked if I'm having another but have never had a negative reaction when I've said no (well, not to my face anyway!). Our decision is partly due to fertility reasons - we tried and failed for another, and don't want to try again - so if pushed, I would tell them that, which would soon shut them up!
Personally, the bereavements/crises issue doesn't worry me - if we have done a good job in raising our son we will have equipped him with the ability to form strong secure friendships and relationships, and those attachments will support him when he faces life's issues. I have a brother and would never dream of turning to him if i needed support - I would rely on my DH and friends. And there is a lot you can do as a parent to ensure you don't become a burden on your child(ren) - forward financial planning, proactive and early downsizing to assisted accommodation, embracing outside agency help, funeral and end of life planning etc.
Purplefeet like you, my only reason for having another would be for my son. But then I know I wouldn't be such a great mum to him if I had another - I'm a SAHM/WAHM and we have the best time together, but if we added a baby to the mix, I know I wouldn't be a very good mum as my resources would be spread too thin.
But I still wobble occasionally, as we're nearing the 'now or never' (for us, in terms of sibling spacing) phase. Hopefully once we're past that the wobbles will become less frequent - it's having the potential to change everything with a simple decision (and consent form!) that I find unsettling.