In fact even being 100% sure itself probably isn't enough, but I for one will at least stop second-guessing you, since being able to perfectly predict who will succeed in advance is basically impossible.

This is a big life decision and it's exactly the wrong time to be a romantic about it. Trying and failing for a PhD is hugely expensive in opportunity cost. Trying, succeeding, and depending on a faculty post on the other end to make it all worthwhile can be pretty dangerous, too.

A lot of us in the 15-35 age range have been force-fed the idea growing up that encouragement is automatically good, regardless, and the entrepreneur community can be especially prone to this because you need a lot of drive to be an entrepreneur. I actually think that's ok for entrepreneurship, because the worst case isn't really all that bad unless you are really stupid. (Incorporate early, incorporate often; don't take on debt personally.) You at least get some very valuable experience. That's not true for everything. Getting your PhD is one of those things; a partial PhD is not worth it. I would be appalled if I encouraged someone to get their PhD and turned out to be that little tipping factor, only for them to find two years in that they are miserable and can't finish. If anything, a little light discouragement is what is really called for; if that's enough to turn you away from a PhD, then good! You were in trouble anyhow.

Life advice is serious business and unthinkingly spouting "common sense" can be very destructive.

But typing all this out is certainly less... pithy.