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.
Most old mining gear does not break even with electric cost so when the blocks half the amount the equipment earns will be cut in half. Will the equipment still run ...most likely but you will be better off to buy the coin instead of the electric.