1. The rule of thumb I've always heard is that $2 million in the bank lets you live off the interest with a salary of $50k/year. Which is all that anyone really needs. $4 million for $100k/year, etc. These are the standard "I'm set for life" numbers, as long as you don't do silly things with your money. Which many many people do (trust fund kids, professional athletes).
2. Just because you can retire doesn't mean that you're going to retire. One of the ways I like to think about it is that an extra $50k/year from my investments can turn a $20k/year job into a $70k/year job. So now all of the sudden I can take all kinds of jobs that I wouldn't have considered doing in the past due to low salary. Plus you don't need the job, so you can walk away from stressful or unpleasant situations much more easily.
3. Personally, I think the remorse that I would feel if I had lost it all due a rogue wave would outweigh the remorse that I'd feel if I had sold the business "too early" and missed out on doubling my money.