That would be breaching copyright. Depending on the company and the product, you'd get anywhere from a pretty nasty debt to straight up ruining your whole life (and potentially the lives of your family and people associated with you).

The same way you wouldn't steal from the mob, you would not steal from a company that makes money on a product FOSS can't compete with. Aside from that, decompilers exist for a very long time yet we have not witnessed such vigilantism.