A lingua franca isn't controllable. French was the lingua franca as it had been the dominant language of trade. Then the British Empire and later USA emerged and dominated global trade, and it became the lingua franca through shear necessity.
In the tech age, English has also become the lingua franca which is likely to cement it's position into the future. In Europe, it's been a convenient second language for many as it allowed Europeans to compete in global trade and also talk to each other with 1 common language, also avoiding nationalist concerns around language. English has also been less controversial as a second language than everyone learning French or German for example given the history of previous european wars.
A language isn't owned by any country, so it doesn't matter that the US is going crazy or that the UK left the EU. English is likely to stay the lingua franca in the west and in Europe as so many people already speak it, it's already well established in schools and culture and in all honesty there isn't an obvious alternative.
In terms of economics, China is powerful but Chinese is spoken largely by one country, and is hard for Europeans to learn due to how fundamentally different it is. India is emerging as an economy, with English it's own lingua franca in a continent divided by numerous languages. Urdu is being pushed by the hindu nationalist government but the global reality is that speaking english is a strength for Indian citizens in trade and global work place, so it's unlikely people will stop learning and speaking English in India in the foreseeable future.
The only other viable alternative in global terms currently for Europe would be Spanish due to the shear number of native speakers. But the problem remains that most Europeans don't speak Spanish and while there is a large number of spanish speakers, they are heavily concentrated in the Americas. Meanwhile English is already spoken widely in Europe, North America outside of Mexico, India, and many other former British Colonies including widely in Africa, Oceania and across Asia.
It's certainly possible things may change, but at the moment it seems unlikely. We're not seeing a huge trend of people moving away from English. One possibility though is that translation apps become near instantaneous and people move away from learning any 2nd language. However I personally think that is unlikely as a translation app can never be perfectly instantaneous due to the nature of grammer - you need the whole of a sentence to translate into another language with a totally different sentence structure, especially for longer and more complex sentences.
So I think it's unlikely English will be displaced as the lingua franca. It is also unneeded - it benefits Europe that a European language is the lingua franca (regardless of the UK exiting the EU etc), and it also benefits Europe as so many Europeans speak English - so the best thing for Europe is to help spread English, and offer a different influence and culture from the US with other English speakers particuarly in emerging economies. English can be Europe's trojan horse for sharing it's culture and values.