When you get to the older tracks, like Monaco, there is plain no room for them.
There have been suggestions of multi-class running as in other classes, but I'm not in favour of this either.
Ferrari in particular have asked to run more cars, which while not ideal, seems to me to be the best option if new teams cant be brought in. Red Bull are in effect doing it anyway. I suppose what used to be the Ferrari test team would be capable of putting a car with enough difference to the main team, as would some of the other teams, but again, they have to be garaged somewhere as well as administrated.
I have nothing to offer but blood, oil, gears, and sweat.