I don’t see Mandarin taking over English as the default language, so to speak. Mainly because it’s way easier for people who know languages like Spanish, German, other European languages to pick up English as a second language then it would be for them to learn Chinese or another Asian one.
https://en.m.wikipedia.org/wiki/List_of_languages_by_total_number_of_speakers#Ethnologue_(2024)
The stats seem to indicate that English is the most spoken language still, though Mandarin wins on first language speakers by far which makes sense.