1. React didn't invent SPAs and reactivity.

2. Ecommerces are not highly dynamic pages. They are overwhelmingly static content with an occasional configurator/cart/search. All things that can be embedded with whatever library you like (including React), or even better none at all.

3. Seo and performance is what really matters in ecommerces. The only minor exceptions are shops like Amazon or Airbnb, but that's unrelated to their seo and performance.

4. I've been writing React and ecommerces using React and similar with millions of daily users for a decade :)