Is it? Maybe. Maybe not. Our desire is largely disconnected from our reward mechanism. Dopamine will cause an intense drive to reach a goal, but once the goal is met, the faucet is switched off, and unless you derive a reward from the goal in itself, you're not guaranteed a feeling of wellbeing at all.
In other words, often we will agonize over cravings for a long time, only to find no joy when they're fulfilled.
Maybe that second marshmallow really would provide enough of a reward to be worth 15 minutes of potentially agonising wait. Or maybe not.
Some persistence is clearly worth it, and in this case the experiment did predict material success later in life. It's then down to what you consider "success" to what extent this is a good thing.
I don't believe they measured happiness for example, and happiness is notably not tied to a great extent to material success (people who don't have shelter and food are substantially less happy than average, but people who are wealthy are only slightly happier on average than the rest of the population - happiness is largely tied to other factors).
Looking at material success it is no wonder that those who are able to resist cravings does better in life. But they are also able to resist pleasures that might very well not be compensated for in any way by whatever goals they are chasing. It is not a given that this tradeoff is good for them. I'm sure some managed to strike a good balance, in both groups, and I'm sure some of the kids who did not have good impulse control ended up doing quite badly because of it. But I'm equally sure some of the kids with good enough impulse control ended up miserable because of that.
Maybe a 15 minute wait for two marshmallows was better for these children than one right away and no agonising for 15 minutes, but I doubt I'd see it that way for my part: I'd see it as a single free marshmallow and get out of there. A second one would be welcome, but it provides far less pleasure than the first one. And 15 minutes to enjoy life rather than trying to keep my mind of something I crave would certainly provide far more pleasure.