Did you not notice that I specified “developed-country Gen Z above” and also China? That was to leave a carve-out for the still very vibrant everyday multilingualism of the Indian Subcontinent, sub-Saharan Africa, etc. But for East Asia, Europe and most of Latin America, the trends speak for themselves. I am not monolingual, nor are many educated people of my generation, but younger people in my country are likely to learn only English alongside their native language (and then stop being curious).

Someone who can speak English on top of their mother tongue is already multilingual. Anecdotally, people who live anywhere close to a border in Europe tends to speak at least two languages, often more than two, regardless of class or profession.

In Latin America, most countries speak Spanish (with the obvious exception of Brazil and smaller colonies from the other European countries), so the every day pressure to learn another language isn't there and English becomes the "obvious" choice. I don't quite get why you seem to discount English entirely.

There's always been a Lingua Franca. It hasn't always been the same one. There will likely always be one.