Your parents already bought late... Those houses were $200k not too long ago, then they were up $500k... about 10 years later, it's $1 million, then your parents took over, then it's $2 million.
How would you know where the peak is? That is one issue.
The second issue is, how do you cash out and still enjoy it? There is no way. You can sell and lease back... but those investors will probably want $5000 per month rent for $2 million home. Would your parents want to pay $60,000 per year just to maintain the same residence? Would your parents be ready to move whenever their landlord needs it for personal use or sell it?