The $75 billion number comes from a study available on SSRN that did an analysis of data from Ethereum, Bitcoin, and Tron: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4742235. The study did not just look at reported victims, they basically started with a large sample of victim addresses and from there tried to derive the scammers' addresses and victim payments that were not reported using a handful of heuristics. You're right that it's over multiple years but even according to the study about 3/4 of the losses were from 2021-2022 if you look at Figure 9.

At 50% of GDP there would be serious economic distortion that would be reflected in statistics somewhere. There would probably at the very least be a massive countrywide construction boom, and not just of fenced compounds. I'm aware of how the scam works - I'm not denying it's huge, I just think those numbers are implausibly large.