So, the only service you really need is something like Tether, not an exchange (as you can use decentralized exchanges). The service Tether operates--and I'm not claiming they aren't also some horrible scam, but I'm using them as the example because they are seemingly the minimal thing you need--is to act as the connection between fiat banks and decentralized chains, by supporting wire/ACH on one side and issuing a token on the other side (which they will later accept back for money). You don't need crazy complicated exchanges with their massive attack surface: you just need Tether. Now, is it possible to have a transparent Tether that you reasonably trust to be legitimate? I don't know. Maybe that's Circle--the people who do USDC, and which have been managing to compete with Tether for the most used stablecoin as of this year--as they do regular SEC filings and have fancy accounting reports? Alternatively, maybe if some large existing bank--such as Bank of America--were willing to issue a stablecoin, we'd all be willing to trust it to not be corrupt? But you really, honestly, don't need a full-blown centralized exchange for crypto to work: you just need a working stablecoin with enough easy-to-access liquidity that you can quickly buy in and out with a ton of money.