The NLRB guidance (https://www.nlrb.gov/strikes) says that economic strikers "can be replaced by their employer", and are "entitled to be recalled to jobs for which they are qualified when openings in such jobs occur", but are not entitled to immediate reinstatement if the employer has hired bona-fide permanent replacements.

I do agree that informal coordination can do much of the work a union would. You just have to convince the company that it'd be a huge headache to replace everyone, because the actual organization work (which those four investigations you describe were about) is unconditionally protected.