Depends on the transaction fees associated with those transfers. If those millions of people also post higher-than-average transaction fees, then the Bitcoin network prioritizes those payments, and cannot process any other transfers.

They would have to wait a lot. Also a fee is almost $20 now. So they would make a bad and slow deal.

That seems like a natural rate limiter that would prevent wasting any meaningful amount of energy on bitcoin.

That transaction fee will be used to run more miners and thus increase the energy usage.

Higher fees means fewer transactions. Nobody's going to buy a $3 coffee with $300 fees just because it's bitcoin.

Nobody will pay a coffee with BTC, just like you don't pay for your stuff with a gold coins.

Some say Litecoin will be used for that but I doubt it. Coins like Cardano or IOTA will be used for transactions. I'm not sure about Cardano but with IOTA you can transfer a fraction of a cent (or no value at all) without fees (that doesn't mean everything will be always fee-less).