1. English became the lingua franca right when the world really became globalized. So everyone from Europe to Asia to Africa has wanted to learn English as a second language for decades. So even if American power went away, I still don't see English falling from its perch. I often say it's really hard for Americans to learn another language because if you go to another country hoping to learn that language, so often you'll find many/most people just want to speak to you in English.
2. The only other power I could see surpassing the US in the mid term is China (and that's in no way guaranteed), but the Chinese language (Mandarin), and especially Chinese writing is inherently more difficult for foreigners to learn. I'd also argue the Chinese writing system is inherently more poorly suited to the digital world.