If another currency took over btc in m.cap right now, it would be big news and a surprise to allot of people, and it would be a big news to write about for many news agencies. This way, it would probably get allot of attention and pump even more by people that hadn't bought before, but bought when they saw the news report it. But if this was to happen in a recent time(1 year), I think it would drop and btc will keep its spot.