He’d be attacked, but not by me. Likewise for Harris or anyone else running for President. I’d call it stupid and probably scammy, but I wouldn’t suggest that it somehow undermines the US.
If Trump’s crypto platform or “Biden Bucks” or whatever takes off enough that it threatens the national currency, then perhaps we should be using it instead of the national currency. Competition is good, and we shouldn’t be using a currency just because the government wants control, we should be using it because it provides value. In the case of the dollar, that value is stability and ubiquity, and if Trump, Harris, or anyone else can do it better than the Federal Reserve, perhaps we should all be using that instead of the dollar. I highly doubt that’s the case, but it certainly doesn’t affect someone’s eligibility to run for President.
That said, they should divest themselves of any controlling interest in anything that could impact their role as President. That means handing off control to someone else, and ensuring that someone else doesn’t have any way to impact your decisions.