But I think most YC founders will be saying "yes" to offers that are in the millions. As pg says "That first million is just worth so much more than the subsequent ones"
When you walk away with a few million dollars in your pocket there's not much stopping you from creating a new startup, if you have the drive.
At startup school they talked about (presumably VC backed) startups being a %15 chance of success. IF you have a %15 chance of building a company worth $20M in 2 years, and you'll get $2M of that-- then the opportunty value is 0.15X2M or $300,000.
On the other hand, if you bootstrap, and you build the same business, your chances of being a success go up to %60 (No flatline VC sitting on your board forcing bad decisions) but maybe you spend a bit less on marketing and grow more organically (really, I'm being generous here- I think the same business would grow just as fast bootstrapped.) so you end up being worth $10M in two years. Your share of that is $5M, for an opportunity value of $3,000,000.
In other words, you're likely to return ten times the money by forgoing venture capital.
While in both cases you could end up with nothing, weighting the payoff with the likihood of success lets you evaluate two very different opportunities.
If you take the choice tht has a %15 chance of success, odds are you won't get that first million... where as in the alternative, odds are you will.
If you have something good and don't need a lot of money there's no reason you have to put yourself at the mercy of VCs -- you can maintain control.
You can't judge this kind of thing in a vacuum. What kind of scenario are you thinking of?
Also, you're assuming that doing your startup on the side "massively decreases your chance of success". I think this is in error-- success is goverened by determination more than anything else, and someone who builds something on the side is clearly determined.
Personally, its more fun to focus on your project completely, and so, its probably psychologically better to work for 3 or 6 months, and then take a 3 or 6 month "break".