No other "International languages" fulfill the characteristics needed for a proper "IL". They're already an indissoluble part of a national culture; therefore, have no neutrality; They're difficult to learn because of a complex grammar, a great number of irregularities and complicated non-neutral pronunciation , which puts you on disadvantage with regard to native speakers, to name a few.
The growth in the usage of English is likely only temporary. The United States is becomingly increasingly rivaled by China, and many international schools are now teaching English and/or Chinese as a foreign language. It is possible that English will be replaced by Chinese. A permanent, official language, would resist changing international influence.