having a monopoly is not illegal. Using a monopoly to stifle competition is.
Actually, you don't need a monopoly to exploit market power, nor even a cartel (itself usually a virtual monopoly). Maybe the Commission might like to look into the UK mobile market then, where the incumbent (vertically integrated) players all own substantial but captive challenger brands pretending to be MVNOs. And in some cases there's a compelling appearance of stifling competiton through predatory pricing through selected resellers, even though the same deal on the captive MVNO's own website is precisely twice the price the reseller offers.