As Mdawg noted El Dorado was forced to sell properties in other markets where post-merger it would have been too dominant. So broadly speaking anti-trust potential in gambling markets exists.
As for the Strip in particular I would say it is a distinct market from downtown or off-Strip. Past a certain and not terribly high price point almost nobody is deciding between downtown and the Strip, despite what classy guys like redietz think about the Circa.
Where the line is drawn IDK.