Hard to imagine it's a scam, running for so many years and I also saw videos of the miners.
Not at all. It's a perfect example of how a successful slow burn Ponzi would operate, the crypto version of Bernie Madoff.
1) There is "proof" of real activity, i.e. the vids to which you refer but it is impossible to know, and you bet Genesis would never allow it to be known, how much hash they have sold against how much hash they actually have.
2) Non refundable cash purchases of contracts, with repayments/ROI at the complete discretion of Genesis and the demon "void if unprofitable" clause in the contract. "Unprofitable" according to Genesis solely.
3) A "good reputation" and trust because everyone knows who Marco Streng is, means premium+ prices are paid for hash. Why should they bother to buy their own equipment? Just sub contract it.