May I remind you of something called "HALVING" before going crazy on a purchasing spree?
That is a million years away. By the time halving arrives, each and every S7 will have long been tossed in the bin. Miner lifetime is far shorter than that.
Why would no one be running S7's when halving occurs next year? I still got all my S3's hashing away. Just cannot bring myself to turn them off. I am still getting 2.7TH out of those old units.