I think you're taking a narrow view of the points being made here. Overall, the biggest benefit is learning another language and learning about another culture in a way that mimics being inside it (during school, anyway).
As for your points about transitioning to all-English schools, I'm confused. All the French immersion schools around here (Northern CA, like you) all bilingual, not French-only (unless I've missed one?). They have to be, for a variety of obvious reasons, such as high school graduates wouldn't meet the admission requirements for UC and Cal State without classes in English and US History.
Plus, the K-8 schools all transition to 50% instruction time in English relatively early on. My DS's school (and others; maybe all of them?) teach math in both languages. PLUS, my DH did school in English and then moved to Europe as a young teenager. Math was the SOLE subject he had no trouble with because it's universal. Etc. So I'm confused about your claim that your niece and nephew had trouble with science and math because of French immersion. TBH, it doesn't make sense as presented (maybe you left something out?).
