The donor is nowhere close to the impact that Genghis Kahn had on the Y chromosome. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1180246/ - and that was in the 11th century with a total word population of about 400M.

A few dozen children from one father today, in a world with a population of 7.5B should not be any concern.

Yeah but that was also an anomaly, which isn't the same as repeatedly decreasing genetic diversity over a number of generations.

On a global scale, not at all.

Locally, given that some of them had already met each-other without knowing they were half-siblings. Maybe a little.

There are over 7 billion people. I think we are ok.

I don't think you really understand how quickly these things propagate.