Very true but that can work both ways. Had the Rams hit the ground running and built a comfortable lead early, that Green Bay contingent would have been silent and that's almost as satisfying as your own fans cheering. Unfortunately for the Rams, this was the Packers Super Bowl this year. They had a bye week to prepare and they gave it one hell of a shot.
I'd love for the Coliseum to be sold out with Rams only fans but I don't think that's a possibility. Does even USC have Trojans only sellouts? Regardless, the Rams have work to do to become that type of nationally revered team. A nomadic franchise with just 3 sporadic championships over the course of 75 years isn't going to garner a devoted fan base. Also, L.A. pride isn't really a thing. Not like Texas for example. LA fans are frontrunners and trend followers. If the team is hot and winning, there will be celebrities and sellouts. But the Rams haven't delivered that in a long time. If they go to 10 Super Bowls and win at least 5 of them, the attendance and support will be a lot stronger.
But the reality is, the Rams haven't delivered a championship to LA since 1951. Considering that kind of drought, even though they are hugely improved and trending upwards, the crowds aren't going to believe for a while.