...it's not "astounding" and you shouldn't have laughed in that hypothetical. Star Wars has never existed as the lone blockbuster franchise and Avengers would have put up the 5th and 7th highest grossing films of all time by the end of TFA box office run (what I assume you mean by "2015").
possibly double
worldwide or domestic? If we're talking worldwide it wouldn't have been crazy for a bullish projection to assume Star Wars would continue to be massive in "traditional" markets and struggle in places like China (or at least never reach a top box office tier in those categories).
The idea that a Star Wars film would end up mirroring Black Panther/TLJ wouldn't be crazy, especially if you're talking pre-TFA release, and the idea of an Avengers film being the biggest thing since sliced bread wouldn't be crazy.