No it’s not a monopoly. Think back to when regulators let Disney to buy Fox even though Disney was absorbing its competitor. From an anti-trust pov, Netflix buying WB is a vertical integration.
First, Netflix owns one facility for its studio, it rents production space and hire contractors. WB owns two major studio facilities (one in California and another in UK) and rents out its own studio so makes money off its own asset. In contrast, Netflix rents space from WB for production, like the Sandman series.
So this possible merger is closer to a vertical integration rather than absorbing a competitor.
Second, a combined Netflix/Warner/HBO/Discovery will result in about about 34% of the streaming market, therefore will be them ahead of ahead of Prime Video (22%) and Disney+/Hulu (21%). So again, not a not a monopoly.
What Netflix wants is Intellectual Properties and franchises, which WB has the largest library. The merger would make Netflix a legit competitor to Disney across the board: production, distribution, and franchises, making Netflix a peer of Disney. Where D+ streaming was Disney using their weight to push into Netflix’s home turf, this merger is Netflix using their cash to push into Disney’s home turf.