Rugby EASILY over soccer, for all the reasons I noted in my earlier post in this thread.
BTW, when Cal did drop football for rugby earlier in the 20th century, crowds for the rugby games were excellent. Give people more exposure to rugby, particularly if football were to cease, and it would be a natural.
But football isn't going to disappear any time soon. It will, however, continue to decline. At Cal it may get a little shot in the arm if the team does well (which I expect it to do). The oncoming victories will produce a surge in attendance because Cal fans have been starved for a winner and some will see victories as a reason to go to CMS (referring to the fair weather fans, of which there are too many). That will allow attendance to increase at least for a period.
Overall, there are few events with the pageantry, the color and tradition of college football. Rugby is a great game......in my opinion better than football. But it doesn't lend itself to halftime shows, pregame shows, and the same kind of in-stands activities for a student body that football provides. The color and pageantry of the college football event is what will save the game. Nothing else compares.