it wouldn't make much sense right now to rent a flat in a monthly rent that is only composed of BTC
For a bitcoiner, it would absolutely make sense to rent a flat, or any other property just for BTC, with a contract priced in BTC instead of fiat, absolutely independently from the exchange rate. I would love to do it, if I could. If you're producing BTC, let say that you have a mining operation and/or any other business and/or investments which yields you a roi in pure BTC, then you don't need to be that concerned about the exchange rate. Obviously, you'll have some expenses in fiat too, but if you reached a point in which you can rent a property for BTC, you surely already know how to manage those very well...
(e.g. a flat that would typically cost 1000 USD/month for 200 BTC/month). As soon as you say that you change the price depending on the BTC <--> fiat exchange rate, you're no longer renting for bitcoins, you're renting for USD but can get paid in BTC.
You don't do it that way. If you rent for BTC, you valuate it in BTC. You can't valuate a contract in BTC based solely on the current price on fiat and the exchange rate. You already have some assumptions of where BTC is headed, so you use that to make a pure BTC valuation which factors in all the particularities, expenses and risks you'll need to handle, including but not limited to just your expenses in fiat...
Same goes for rent etc. - even if it's PAID in BTC, it doesn't mean it's PRICED in BTC. Bitcoin is still highly volatile, has a huge inflation compared to fiat currencies and even though they are easier and easier to be used, most products one can buy are priced in fiat in the end and won't keep their current prices should the exchange rate jump in one or the other direction.
We are all well aware of those particularities of BTC, and I think that we are here because we've managed to handle them until now. Obviously an agreement to rent a property in pure BTC would be risky for both sides, but as a renter you'll be hedging that risk renting other properties for fiat, maintaining a healthy buffer of any currency you need, and with any financial derivative you feel comfortable with...