Can you walk us through the specific integration points, data dependencies, stored data, or established workflows that make it genuinely hard for an employer to leave eRecruiter?

I don't think that, especially two to three years ago, there were many points routing customers to eRecruiter. The entire database was on eRecruiter's servers, so access to those candidates and CVs was only possible through our tool. However, every customer was, and I believe still is, able to download the entire database. Apart from the habit of using eRecruiter, I wouldn't say there were many obstacles blocking customers from leaving.

For those above 20, is an ATS a must as it brings a high return on investment—material and quantifiable efficiencies in HR personnel? Or is it a nice-to-have, mostly about better hiring quality results?

For companies that don't want to spend the money, the level where it becomes a must rather than a nice-to-have is around 50 recruitments, because at this level it's difficult to run that number of processes without a proper tool. When you start thinking about tools, you have choices and can select from different solutions on the market. eRecruiter was always favored because of its close connection with Pracuj.pl.