10 or 20 years from now, when bitcoin's market cap equals or surpasses the gold's market cap, we will look back and think how lucky the people who bought bitcoin when it was under $1,000,000.
The time will also come where we will not talk about buying bitcoin, but instead buying satoshis.
10 to 20 years? I think it's going to be much earlier for BTC to reach $1m. Especially at the pace adoption is growing lately. Institutional investment companies continue to buy BTC non-stop, while governments are quickly exploring the idea of a national Bitcoin reserve. El Salvador even went as far as making BTC legal tender. Just wait a few more years, and Bitcoin will finally reach said milestone. Not only that, but it will also be able to surpass Gold in market cap.
At such high prices ($1m per BTC), I'd expect developers to divide satoshis into smaller fractions (aka fractional satoshis). You're right that we'll be talking about buying satoshis instead of bitcoins in the future. The Lightning Network even uses satoshis (sats) as a standard unit of account. People will probably use the LN more throughly for daily payments. The future is full of surprises, so anything can happen.