That's too broad a statement. There are some theories in economics that work very well, like supply and demand, or arbitrage theory. The idea of the cost of capital is also a fairly well-trodden one, it is taught in business courses, used on wallstreet, and seems to work in practice.

I can't think of a single broad economic axiom or theory that doesn't depend—wholly or in-part—on the principle of ceteris paribus, which excludes their practical application to essentially everything in real life.

In a few cases, economic theories do a passable job of roughly mirroring real-world behavior, but that's the exception and not the rule.

>on the principle of ceteris paribus, which excludes their practical application to essentially everything in real life.

When a principle is violated marginally, conclusions made based on the principle are themselves violated marginally. You can't write off chemistry because relativistic effects violate the conservation of mass any more than you can write off economics because no two soup cans are identical.

Economics waves off secondary effects at an entirely different level than Chemistry.