You need only look at the makeup of every MBA, MEng, MS, and STEM PhD program. 50% European and 50% Asian, all foreigners (i.e. immigrated on a student visa). If there was no brain drain then you'd expect mostly US permanent residents/citizens.
Brain drain doesn’t mean that all top talent is gone, but rather that there is less of it than there would have been otherwise.
That this is happening between Europe and US is beyond clear: European engineers often emigrate to US, but American engineers emigrate much more rarely to Europe.
You need to back this up with something better than anecdotal evidence - FWIW I've worked with plenty of world class American engineers who don't live in the US