If that's their whole retirement investment (as they said it's their only income, no idea about us retirement details) if they don't increase your rent, their net income will GO DOWN. Prices of everything also went up for them, if you think it's hard with constant income, imagine with declining income.

The value of their house going up is useless to pay for bread.

You should get a bigger paycheck, average wage growth is around 5% in the US, higher than inflation even.