With low electricity cost, S9 should be profitable next year as well because not much more power effecient equipment is planned anytime soon, but you should pay only the right BTC so you can realistically mine the Bitcoin back preferably in about half of a year to be safe.

And predicting longterm difficulty is not easy, so you might end up mining the BTC you paid much longer, but obviously you can sell the S9 as well for Bitcoin (but if the price goes up it might be not much BTC anymore). Its risky, but its fun and you can turn Bitcoin profit.