The market cap will also increase as you buy loads of coins. For example if I went on coinbase pro, there are people willing to sell up to 50,000 ETH at a maximum price of $6,000, or a total cost of $220,000 million. Of course Coinbase isn't the only market, but if it was and I bought that 220,000 million dollars of ETH the market cap would now be around 700B, at least temporarily.

My guess would be at some point people would realize a takeover was occurring and panic, but it seems like a 51% buyout would require an ungodly amount of money and time.