I mean, that's what happens as a cuisine gets passed down from generation to generation: it evolves. Cuisine and language are similar in that respect. The sort of English spoken in the US is no less "real" than the sort of English spoken in England; why would the Chinese food made and sold by the descendants of Chinese immigrants in the US be any less "real" than the Chinese food made and sold in China? They both evolved from the same point, and simply diverged and evolved.

And that's what we're seeing now with Thai food. As the generations get further and further removed from the original immigrants, the cuisine continues to evolve in its own direction. And why shouldn't it?

It's also worth remembering that restaurants are not always (and in fact, rarely are) representative of the actual local cuisines. There's still heavy emphasis on the good ol' home cooked meal (as hard as frozen food chains might try to supplant home cooking), and that's where you see both the preservation of tradition and the proliferation of experimentation, marketability and profit be damned. Indeed, looking at these home cooked meals, it becomes readily evident that the oft-denigrated Chinese holes-in-the-wall and small town pizzerias are far closer to authenticity than the fancy gastropubs and fusion restaurants of snobbish preference; authentic cuisine puts form over function, and heartiness over presentation. Authenticity comes from humility and self-awareness; it doesn't need to try to impress anyone's palate.