Employer organisations also consist of multiple employers. In a fully unionised bargaining, both employer and employee unions attend bargaining process.

There are sufficient number of software and database companies to establish an employer organisation for software.

Fairness depends on the ratio of employee employer monopoly power. There may be enough tech companies to form a union, but that just means employees need to be more consolidated to match. Your "fully unionized bargaining" case is just one point in the blanced subspace. A many-to-many relationship where the employers per employee matched the employees per employer (along with other hierarchical symmetries) would be an (outlandish) other such point.