Not really sure what you’re getting at — no way around it man, you left it late. A half-assed plan B will always be exactly that.
By just citing BU you are inherently cherry picking. Look at CMU MSCF, NYU Courant — far more representative of what demographics are like program-to-program, at least for top ones — and you’ll see many US undergrads. Sure, many are international students that came to the US for undergrad, but if this still bothers you than I think you’re choosing the wrong field. If you go to a top US school for undergrad, and are not pursing law/medicine/PhD, why in the world would you do a professional masters, it’s simply not necessary for the well-trained student coming from a good institution planning to pursue a career sufficiently related to their undergrad major. You can find plenty of US citizens that are quants and did not pursue an MFE-type degree, however, note where they went for undergrad. If you go to a lower tier school, you likely are only going to be exposed to quantitative finance if you go sniffing on your own because (i) career services are nonexistent and (ii) less likely you have available courses related to quant finance to spark your interest. Thus, most students end up pursuing something more generic and widely known, such as data science. I think both of these things, combined with the fact that MFEs are a popular way for international students with non US undergrad to move to the US quant job market, are two primary factors leading to not many US citizens in MFEs.