So what do you think would happen if another Cryptocurrency (Example Monero / Ethereum etc) were to take Bitcoins spot as first in market cap?
Well to reach that state, that means that 3 things could have happened.
1) People decided to pull most of their money out of Bitcoin, so the evaluation of it goes down and so does the market cap, allowing for a new coin to come up.
2) More people decided to put their money into a new coin than there already is in Bitcoin, so the new coin rises above bitcoin in terms of market cap.
3) A mixture of both of the above.
In the case that a new currency is seen as the "main" one, it's likely that it'll end up being the one that moves into the future for a while. We haven't really seen any coins that have taken Bitcoin's place, so it's very hard to state what would happen as of now, but I think if a new coin comes up, Bitcoin will go very far down.