If one whale decides to invest more into Bitcoin Cash than Bitcoin's whole market cap, does that mean Bitcoin Cash will replace Bitcoin as #1 for good?
You're delusional!
Market cap doesn't mean much in terms of price. When Bitcoin Cash came out it cloned Bitcoins market cap, so theoretically it could divide Bitcoin's price in two but it didn't. People decided it's not even worth 20% of Bitcoin.
Even if we follow your thinking Bitcoin's market cap is $67,991,537,790. No whales have that kind of money.