There's a pitfall when assuming success while creating big solutions to big problems. You're liable to fall into the trap of origami solutions.
Imagine making an origami crane. You'll carefully follow perhaps dozens of steps. They all must be done in the right order, with the right pressure, the right paper must be selected before you start, in the right shape and size. If you forget a step, or mix-up two steps, or if the paper tears on one particular step, then you'll get to the end, pull two ends of the paper crane, and it's as likely as anything else that the whole mess will fall right apart in your hands.
Now imagine you are solving a business problem. If you assume success, then of course those well-architected plans look doable. But do you really want to bet your business on a series of many intricate steps whose only value can be derived from the flawless serial execution of them all?
When making plans, do your best to realize value at every step of the way, and avoid the trap of betting on heads-down work for months, and hoping a beautiful crane will emerge on the other side.