One of the major manifestations of the human condition is our expectation that anything easy to obtain and/or operate should be "free."
Counterexample: Linux and open source software in general. The money will be in adapting tools to specific purposes, not selling copies of tools which can be duplicated essentially for free.
Ahhh, I see your point. There are conditions to it, however. There will always be the need for competent technical know-how. Even in the situation where just about anything you can imagine can be built doesn't mean there won't be further frontiers that can be explored and/or exploited. Someone has to know how the technology that will make new things works. That's where one frontier will be - pushing whatever future tech has to offer. Future technology will have limitations, just like any other technology adn the engineers and scientists will be there, pushing that envelop back farther.
And if it reaches the point you describe, would it not be more of a stagnation than a decline? Lack of incentive to innovate will limit growth, but why would it cause a retraction of knowledge or technology?