afaict, the main advantages of buying things with crypto are (potentially) lower transaction fees and latencies, and less red tape restricting who can buy what from whom. If you use a "tradfi onramp", doesn't it negate all of that? Why not just use tradfi all the way, for that transaction?

This was thr original intent of thr Satoshi paper, except BTC is probably the worst payment network you can think about, which is why it failed to gain any use as such over the past 18 years…